Document number: P0355R4

Howard E. Hinnant
2017-10-16

Extending `<chrono>` to Calendars and Time Zones

Revision History
Introduction
Description
Issues
Proposed Wording
Acknowledgements
References

Revision History

Changes since R3

Presume that the time zone database is supplied only by the std::lib implementation, but can be updated at run time. Removed remote_download and remote_install. This functionality is now subsumed by the implementation.
to_stream sets failbit if it is required to create a name for an invalid month or weekday.
Add note to promise compatibility with fmt (P0645)
Have format throw an exception if anything happens so that it could not return an accurate string.
Template zoned_time on TimeZonePtr.
Give weekday_indexed a defaulted default constructor.
Make from_stream and to_stream customization points.
Make the database singleton a singly linked list of tzdb with an atomic head pointer, instead of a single tzdb.
Rewrite in terms of string_view.
Improve spec for operator-(const year_month& x, const year_month& y).
Refine constraints on conversions from calendar types to sys_days.
Added zoned_time default constructor.
Added zoned_time deduction guides.
Correct minor type-o's.
Correct html bugs.

Changes since R2

Add to_stream and from_stream to utc_time<Duration>, tai_time<Duration>, and gps_time<Duration>.
Add from_stream, and rewrite format and parse in terms of to_stream and from_stream.
Add to_stream and from_stream for year, month, day, weekday, year_month, and month_day.
to_stream will set failbit instead of throw.
Add file_clock and hook it into filesytem.
Relax zoned_time to be coarser than seconds.
Remove make_time and make_zoned in favor of the implicit deduction guides.
Create zoned_time(const char* name, ...) overloads to enable implicit deduction guides.
Give time_of_day default constructor.
Add Alloc to basic_string everywhere possible.
Deprecate system_clock::to_time_t and system_clock::from_time_t.
Make duration streaming respect padding and alignment requests.
Add is_clock type trait.
Allow the Clock template parameter of time_point to be a Clock or a local_t.
Make functions in [thread] that take a Clock template parameter ill-formed if is_clock{} is false.

Changes since R1

Make time_point incremental and decrementable
Add unary operators + and - to year
Remove enum {am, pm}, time_of_day constructors which use it, and make_time factory functions which use it.
Add to_stream.
Add format and parse for duration.

Changes since R0

Tighten up utc_clock::utc_to_sys and utc_clock::sys_to_utc.
Eliminate utc_clock::utc_to_sys and utc_clock::sys_to_utc in favor of free functions such as to_sys_time and to_utc_time.
Give utc_time a streaming operator.
Change format to take time_points by const& instead of by value.
Add trivial default constructors to most calendar types.
Create %Ez & %Oz to put ':' in offset for format and parse.
Add %F to parse.
Changed parse functions to parse manipulators.
Excuse local_t from being a clock
Guarantee Unix Time.
Add duration stream insertion.
Introduce tai_clock.
Introduce gps_clock.
Removed noexcept from make_time.

Introduction

The purpose of a calendar is to give a name to each day.¹ There are many different ways this can be accomplished. This paper proposes only the Gregorian calendar. However the design of this proposal is such that clients can code other calendars and have them interoperate with <chrono>, the civil calendar, and with time zones, all with a minimal coupling. For example:

#include "coptic.h"  // not proposed, just an example
#include <chrono>
#include <iostream>

int
main()
{
    using namespace std::chrono_literals;
    auto date = 2016y/may/29;
    cout << date << " is " << coptic::year_month_day{date} << " in the Coptic calendar\n";
    // 2016-05-29 is 1732-09-21 in the Coptic calendar
}

The above example creates a date in the Gregorian calendar (proposed) with the literal 2016y/may/29. The meaning of this literal is without question. It is conventional and clearly readable. This proposal has no knowledge whatsoever of the Coptic calendar. However it is relatively easy to create a Coptic calendar (which knows nothing about the Gregorian calendar), which will convert to and from the Gregorian calendar. This is done by establishing a clear and simple communication channel between calendar systems and the <chrono> library (specifically a system_clock::time_point with a precision of days).

The paper proposes:

Minimal extensions to <chrono> to support calendar and time zone libraries.
A proleptic Gregorian calendar, hereafter referred to as the civil calendar.
A time zone library based on the IANA Time Zone Database.
strftime-like formatting and parsing facilities with fully operational support for fractional seconds, time zone abbreviations, and UTC offsets.
Several <chrono> clocks for computing with leap seconds which is also supported by the IANA Time Zone Database.

Everything proposed herein has been fully implemented here:

https://github.com/HowardHinnant/date

The implementation includes full documentation, and an active community of users with positive field experience. The implementation has been ported to Windows, Linux, Android, macOS and iOS.

The API stresses:

Seamless integration with the existing <chrono> library.
Type safety.
Detection of errors at compile time.
Performance.
Ease of use.
Readable code.
No artificial restrictions on precision. Note: current financial software is currently in the middle of a transition from seconds precision to milliseconds precision. This library handles that transition as seamlessly as <chrono> does.

Listing "Performance" in the API design deserves a little explanation as one usually thinks of that as an implementation issue. Think of it this way:

If vector<T>::push_front(const T&) existed, that would encourage inefficient code, even though it would be trivial to implement.
If list<T>::operator[](size_type index) existed, that would encourage inefficient code, even though it would be trivial to implement (by incrementing from begin() or decrementing from end()).

This API makes it convenient to write efficient code, and inconvenient to write inefficient code. It turns out that conversion between a field type such as {year, month, day} and a serial type such as {count-of-days} is one of the more expensive operations when dealing with calendrical computations. Both data structures are very useful (just as both vector and list are very useful). So this library puts you in control of when and how often that conversion takes place, and makes it easy to avoid such conversions when not necessary.

Description

One can create a year like this:

auto y = year{2016};

Just like <chrono>, type safety is taken very seriously. The type year is distinct from type int, just as 3 can never mean "3 seconds", unless it is explicitly typed to do so: seconds{3}.

And just like seconds, there is a year literal suffix which can help make your code more readable:

auto y = 2016y;

year is a partial-calendar-type. It can be combined with other partial-calendar-types to create a full-calendar-type such as year_month_day. Full-calendar-types can be converted to and from the family of system_clock::time_points. Full-calendar-types such as year_month_day are time points with a precision of a day, but they are also field types. They are composed of 3 fields under the hood: year, month and day. Thus when you construct a year_month_day from a year, month and day, absolutely no computation takes place. The only thing that happens is a year, month and day are stored inside the year_month_day.

year_month_day ymd1{2016y, month{5}, day{29}};

This is a very simple operation and can even be made constexpr when all of the inputs are compile-time constants. And conventional syntax is available which means the exact same thing, with the same run-time or compile-time performance. It can make date literals much more readable without sacrificing type safety:

constexpr year_month_day ymd1{2016y, month{5}, day{29}};
constexpr auto ymd2 = 2016y/may/29d;
static_assert(ymd1         == ymd2);
static_assert(ymd1.year()  == 2016y);
static_assert(ymd1.month() == may);
static_assert(ymd1.day()   == 29d);

year_month_day is a very simple, very understandable calendrical data structure:

class year_month_day
{
    chrono::year  y_;  // exposition only
    chrono::month m_;  // exposition only
    chrono::day   d_;  // exposition only

public:
    constexpr year_month_day(const chrono::year& y, const chrono::month& m, const chrono::day& d) noexcept;
    // ...

By now you should be yawning and muttering "so what?"

Now we introduce a little <chrono> infrastructure that serves as the communication channel with simplistic calendrical data structures such as year_month_day.

using days = duration<int32_t, ratio_multiply<ratio<24>, hours::period>>;
template <class Duration> using sys_time = time_point<system_clock, Duration>;
using sys_days = sys_time<days>;

sys_days is a std::chrono::time_point. This time_point is based on system_clock and has a very coarse precision: 24 hours. Just as system_clock::time_point is nothing more than a count of microseconds (or nanoseconds, or whatever), sys_days is simply a count of days since the system_clock epoch. And sys_days is fully interoperable with system_clock::time_point in all of the ways normal to the <chrono> library:

sys_days implicitly converts to system_clock::time_point with no truncation error.
system_clock::time_point does not implicitly convert to sys_days because it would involve truncation error.
Explicit conversion can be achieved from system_clock::time_point by using the existing <chrono> facilities time_point_cast or floor.

constexpr system_clock::time_point tp = sys_days{2016y/may/29d}; // Convert date to time_point
static_assert(tp.time_since_epoch() == 1'464'480'000'000'000us);
constexpr auto ymd = year_month_day{floor<days>(tp)};            // Convert time_point to date
static_assert(ymd == 2016y/may/29d);

The calendrical type year_month_day provides conversions to and from sys_days. This conversion is easy to do for std::lib implementors using algorithms such as these. If the committee standardizes existing practice and specifies that system_clock measures Unix Time, then it will be equally easy for anyone to write their own calendar system which converts to and from sys_days (e.g. the coptic example in the introduction).

This proposal actually contains a second calendar. It is so closely related to the civil calendar that we normally don't think of it as another calendar. We often refer to dates like "the 5th Sunday of May in 2016" as opposed to "the 29th of May in 2016." This proposal makes it so easy to build fully functional calendars that interoperate with system_clock::time_point, that it is nearly trivial to include such functionality:

constexpr system_clock::time_point tp = sys_days{sun[5]/may/2016}; // Convert date to time_point
static_assert(tp.time_since_epoch() == 1'464'480'000'000'000us);
constexpr auto ymd = year_month_weekday{floor<days>(tp)};          // Convert time_point to date
static_assert(ymd == sun[5]/may/2016);

The literal sun[5]/may/2016 means "the 5th Sunday of May in 2016." The conventional syntax is remarkably readable. Constructor syntax is also available to do the same thing. The type constructed is year_month_weekday which does nothing but store a year, month, weekday, and the number 5. This "auxiliary calendar" converts to and from sys_days just like year_month_day as demonstrated above. As such, year_month_weekday will interoperate with year_month_day (by bouncing off of sys_days) just as it will with any other calendar that interoperates with sys_days:

static_assert(2016y/may/29d == year_month_day{sun[5]/may/2016});

Since year_month_day is so easy to convert to (or from) a time_point it makes sense to convert to a time_point when you need to talk about a date and time-of-day:

constexpr auto tp = sys_days{2016y/may/29d} + 7h + 30min; // 2016-05-29 07:30 UTC

The time zone is implicitly UTC because system_clock tracks Unix Time which is (a very close approximation to) UTC. If you need another time zone, no worries, we'll get there. And remember, tp above is a system_clock::time_point, except with minutes precision. You can compare it with system_clock::now() to find out if the date is in the past or the future. Also note that the syntax above (like <chrono>) is precision neutral. That's because the syntax above is <chrono>, except for the part converting a calendar type into the <chrono> system. If you suddenly need to convert your minutes-precision time point into seconds or milliseconds (or whatever) precision, the change is seamlessly handled by the existing <chrono> system:

constexpr auto tp = sys_days{2016y/may/29d} + 7h + 30min + 6s + 153ms; // 2016-05-29 07:30:06.153 UTC

Simple streaming is provided:

cout << tp << '\n';  // 2016-05-29 07:30:06.153

But I need the time in Tokyo!

auto tp = sys_days{2016y/may/29d} + 7h + 30min + 6s + 153ms; // 2016-05-29 07:30:06.153 UTC
zoned_time zt = {"Asia/Tokyo", tp};
cout << zt << '\n';                                          // 2016-05-29 16:30:06.153 JST

zoned_time is templated on the duration type of tp, which is automatically deduced from the initialization expression (milliseconds in this example). This effectively pairs a time zone with a time point. In this example we pair the time zone "Asia/Tokyo" with a sys_time (which is implicitly UTC). When printed out, you see the local time, and by default the current time zone abbreviation. Also by default, you see the full precision of the zoned_time.

Sometimes, instead of specifying the time in UTC as above, it is convenient to specify the time in terms of the local time of the time zone. It is very easy to change the above example to mean 7:30 JST instead of 7:30 UTC:

auto tp = local_days{2016y/may/29d} + 7h + 30min + 6s + 153ms; // 2016-05-29 07:30:06.153
auto zt = zoned_time{"Asia/Tokyo", tp};
cout << zt << '\n';                                            // 2016-05-29 07:30:06.153 JST

The only change to the code is the use of local_days in place of sys_days. local_days is also a std::chrono::time_point but its "clock type" local_t has no now() function. This time_point is called local_time. A local_time can refer to any time zone. In the above example when we pair "Asia/Tokyo" with the local_time, the result becomes a zoned_time with the local time specified by the local_time.

To interoperate with time zones, calendrical types must convert to and from local_days as well as sys_days. The math is identical for both conversions, so it is very easy for the calendar author to provide. But as seen in this example, the meaning can be quite different.

The client of the calendar library can easily use the calendar types with the time zone library, specifying times either in the local time, or in UTC, simply by switching between local_days and sys_days. Here is an example that sets up a meeting at 9am on the third Tuesday of June, 2016 in New York:

auto zt = zoned_time{"America/New_York", local_days{tue[3]/jun/2016} + 9h};
cout << zt << '\n';  // 2016-06-21 09:00:00 EDT

Need to set up a video conference with your partners in Helsinki?

cout << zoned_time{"Europe/Helsinki", zt} << '\n';

This converts one zoned_time into another zoned_time where the only difference is changing from "America/New_York" to "Europe/Helsinki". The conversion preserves the UTC equivalent in both zoned_times, and therefore outputs:

2016-06-21 16:00:00 EEST

And if this is not the formatting you prefer, that is easily fixed too:

cout << format("%F %H:%M %z", zoned_time{"Europe/Helsinki", zt}) << '\n';
// 2016-06-21 16:00 +0300

Or perhaps properly localized:

cout << format(locale{"fi_FI"}, "%c", zoned_time{"Europe/Helsinki", zt}) << '\n';
// Ti 21 Kes 16:00:00 2016

Wait, slow down, this is too much information! Let's start at the beginning. How do I get the current time?

cout << system_clock::now() << " UTC\n";
// 2016-05-30 17:57:30.694574 UTC

My current local time?

cout << zoned_time{current_zone(), system_clock::now()} << '\n';
// 2016-05-30 13:57:30.694574 EDT

Current time in Budapest?

cout << zoned_time{"Europe/Budapest", system_clock::now()} << '\n';
// 2016-05-30 19:57:30.694574 CEST

For more documentation about the calendar portion of this proposal, including more details, more examples, and performance analyses, please see:

http://howardhinnant.github.io/date/date.html

For a video introduction to the calendar portion, please see:

https://www.youtube.com/watch?v=tzyGjOm8AKo

For a video introduction to the time zone portion, please see:

https://www.youtube.com/watch?v=Vwd3pduVGKY

For more documentation about the time zone portion of this proposal, including more details, and more examples, please see:

http://howardhinnant.github.io/date/tz.html

For more examples, some of which are written by users of this library, please see:

https://github.com/HowardHinnant/date/wiki/Examples-and-Recipes

For another example calendar which models the ISO week-based calendar, please see:

http://howardhinnant.github.io/date/iso_week.html

Proposed Wording

Table of Contents for Proposed Wording

[time] 23.17
    [time.general]   23.17.1
    [time.syn]       23.17.2
    [time.clock.req] 23.17.3
    [time.traits]    23.17.4
        [time.traits.is_fp]           23.17.4.1
        [time.traits.duration_values] 23.17.4.2
        [time.traits.specializations] 23.17.4.3
        [time.traits.is_clock]
    [time.duration]  23.17.5
        [time.duration.cons]        23.17.5.1
        [time.duration.observer]    23.17.5.2
        [time.duration.arithmetic]  23.17.5.3
        [time.duration.special]     23.17.5.4
        [time.duration.nonmember]   23.17.5.5
        [time.duration.comparisons] 23.17.5.6
        [time.duration.cast]        23.17.5.7
        [time.duration.literals]    23.17.5.8
        [time.duration.alg]         23.17.5.9
        [time.duration.io]
    [time.point]     23.17.6
        [time.point.cons]        23.17.6.1
        [time.point.observer]    23.17.6.2
        [time.point.arithmetic]  23.17.6.3
        [time.point.special]     23.17.6.4
        [time.point.nonmember]   23.17.6.5
        [time.point.comparisons] 23.17.6.6
        [time.point.cast]        23.17.6.7
    [time.clock]     23.17.7
        [time.clock.system] 23.17.7.1
        [time.clock.utc]
        [time.clock.tai]
        [time.clock.gps]
        [time.clock.file]
        [time.clock.steady] 23.17.7.2
        [time.clock.hires]  23.17.7.3
        [time.clock.local_time]
    [time.format]
    [time.parse]
    [time.calendar]
        [time.calendar.last]
        [time.calendar.day]
        [time.calendar.month]
        [time.calendar.year]
        [time.calendar.weekday]
        [time.calendar.weekday_indexed]
        [time.calendar.weekday_last]
        [time.calendar.month_day]
        [time.calendar.month_day_last]
        [time.calendar.month_weekday]
        [time.calendar.month_weekday_last]
        [time.calendar.year_month]
        [time.calendar.year_month_day]
        [time.calendar.year_month_day_last]
        [time.calendar.year_month_weekday]
        [time.calendar.year_month_weekday_last]
        [time.calendar.operators]
    [time.time_of_day]
    [time.timezone]
        [time.timezone.database]
            [time.timezone.database.remote]
        [time.timezone.exception]
        [time.timezone.info]
        [time.timezone.time_zone]
        [time.timezone.zoned_traits]
        [time.timezone.zoned_time]
        [time.timezone.leap]
        [time.timezone.link]
    [ctime.syn]      23.17.8

[fs.filesystem.syn]    30.10.6

[thread.req.paramname] 33.2.1

Text in grey boxes is not proposed wording.

Insert into synopsis in 23.17.2 Header <chrono> synopsis [time.syn]:

namespace std {
namespace chrono {

// ...
// customization traits
// ...

template <class T> struct is_clock;
template <class T> inline constexpr bool is_clock_v = is_clock<T>::value;

// duration I/O
template <class charT, class traits, class Rep, class Period>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               const duration<Rep, Period>& d);

template <class charT, class traits, class Rep, class Period>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt,
          const duration<Rep, Period>& d);

template <class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            duration<Rep, Period>& d,
            basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);

// ...
// convenience typedefs
// ...
using days   = duration<signed integer type of at least 25 bits, ratio_multiply<ratio<24>, hours::period>>;
using weeks  = duration<signed integer type of at least 22 bits, ratio_multiply<ratio<7>, days::period>>;
using years  = duration<signed integer type of at least 17 bits, ratio_multiply<ratio<146097, 400>, days::period>>;
using months = duration<signed integer type of at least 20 bits, ratio_divide<years::period, ratio<12>>>;

// ...
// clocks
// ...
class utc_clock;
class tai_clock;
class gps_clock;
class file_clock;

// time_point families
template <class Duration>
    using sys_time = time_point<system_clock, Duration>;
using sys_seconds  = sys_time<seconds>;
using sys_days     = sys_time<days>;

struct local_t {};
template <class Duration>
    using local_time = time_point<local_t, Duration>;
using local_seconds  = local_time<seconds>;
using local_days     = local_time<days>;

template <class Duration>
    using utc_time = time_point<utc_clock, Duration>;
using utc_seconds  = utc_time<seconds>;

template <class Duration>
    using tai_time = time_point<tai_clock, Duration>;
using tai_seconds  = tai_time<seconds>;

template <class Duration>
    using gps_time = time_point<gps_clock, Duration>;
using gps_seconds  = gps_time<seconds>;

template <class Duration>
    using file_time = time_point<file_clock, Duration>;

// time_point conversions
template <class Duration>
    sys_time<common_type_t<Duration, seconds>>
    to_sys_time(const utc_time<Duration>& t);
template <class Duration>
    sys_time<common_type_t<Duration, seconds>>
    to_sys_time(const tai_time<Duration>& t);
template <class Duration>
    sys_time<common_type_t<Duration, seconds>>
    to_sys_time(const gps_time<Duration>& t);
template <class Duration>
    sys_time<Duration>
    to_sys_time(const file_time<Duration>& t);

template <class Duration>
    utc_time<common_type_t<Duration, seconds>>
    to_utc_time(const sys_time<Duration>& t);
template <class Duration>
    utc_time<common_type_t<Duration, seconds>>
    to_utc_time(const tai_time<Duration>& t);
template <class Duration>
    utc_time<common_type_t<Duration, seconds>>
    to_utc_time(const gps_time<Duration>& t);
template <class Duration>
    utc_time<common_type_t<Duration, seconds>>
    to_utc_time(const file_time<Duration>& t);

template <class Duration>
    tai_time<common_type_t<Duration, seconds>>
    to_tai_time(const sys_time<Duration>& t);
template <class Duration>
    tai_time<common_type_t<Duration, seconds>>
    to_tai_time(const utc_time<Duration>& t);
template <class Duration>
    tai_time<common_type_t<Duration, seconds>>
    to_tai_time(const gps_time<Duration>& t);
template <class Duration>
    tai_time<common_type_t<Duration, seconds>>
    to_tai_time(const file_time<Duration>& t);

template <class Duration>
    gps_time<common_type_t<Duration, seconds>>
    to_gps_time(const sys_time<Duration>& t);
template <class Duration>
    gps_time<common_type_t<Duration, seconds>>
    to_gps_time(const utc_time<Duration>& t);
template <class Duration>
    gps_time<common_type_t<Duration, seconds>>
    to_gps_time(const tai_time<Duration>& t);
template <class Duration>
    gps_time<common_type_t<Duration, seconds>>
    to_gps_time(const file_time<Duration>& t);

template <class Duration>
    file_time<Duration>
    to_file_time(const sys_time<Duration>& t);
template <class Duration>
    file_time<common_type_t<Duration, seconds>>
    to_file_time(const utc_time<Duration>& t);
template <class Duration>
    file_time<common_type_t<Duration, seconds>>
    to_file_time(const tai_time<Duration>& t);
template <class Duration>
    file_time<common_type_t<Duration, seconds>>
    to_file_time(const gps_time<Duration>& t);

// time_point I/O

// operator<<
template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const local_time<Duration>& tp);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const sys_days& dp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const utc_time<Duration>& t);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const tai_time<Duration>& t);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const gps_time<Duration>& t);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& tp);

// to_stream
template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const sys_time<Duration>& tp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const local_time<Duration>& tp,
              const string* abbrev = nullptr, const seconds* offset_sec = nullptr);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const utc_time<Duration>& tp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const tai_time<Duration>& tp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const gps_time<Duration>& tp);

template <class charT, class traits, class Duration>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const file_time<Duration>& tp);

// from_stream
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                sys_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                local_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                utc_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                tai_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                gps_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                file_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

// Calendrical types

struct last_spec;

class day;

constexpr bool operator==(const day& x, const day& y) noexcept;
constexpr bool operator!=(const day& x, const day& y) noexcept;
constexpr bool operator< (const day& x, const day& y) noexcept;
constexpr bool operator> (const day& x, const day& y) noexcept;
constexpr bool operator<=(const day& x, const day& y) noexcept;
constexpr bool operator>=(const day& x, const day& y) noexcept;

constexpr day  operator+(const day&  x, const days& y) noexcept;
constexpr day  operator+(const days& x, const day&  y) noexcept;
constexpr day  operator-(const day&  x, const days& y) noexcept;
constexpr days operator-(const day&  x, const day&  y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const day& d);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const day& d);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                day& d, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class month;

constexpr bool operator==(const month& x, const month& y) noexcept;
constexpr bool operator!=(const month& x, const month& y) noexcept;
constexpr bool operator< (const month& x, const month& y) noexcept;
constexpr bool operator> (const month& x, const month& y) noexcept;
constexpr bool operator<=(const month& x, const month& y) noexcept;
constexpr bool operator>=(const month& x, const month& y) noexcept;

constexpr month  operator+(const month&  x, const months& y) noexcept;
constexpr month  operator+(const months& x,  const month& y) noexcept;
constexpr month  operator-(const month&  x, const months& y) noexcept;
constexpr months operator-(const month&  x,  const month& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month& m);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month& m);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                month& m, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class year;

constexpr bool operator==(const year& x, const year& y) noexcept;
constexpr bool operator!=(const year& x, const year& y) noexcept;
constexpr bool operator< (const year& x, const year& y) noexcept;
constexpr bool operator> (const year& x, const year& y) noexcept;
constexpr bool operator<=(const year& x, const year& y) noexcept;
constexpr bool operator>=(const year& x, const year& y) noexcept;

constexpr year  operator+(const year&  x, const years& y) noexcept;
constexpr year  operator+(const years& x, const year&  y) noexcept;
constexpr year  operator-(const year&  x, const years& y) noexcept;
constexpr years operator-(const year&  x, const year&  y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year& y);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year& y);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year& y, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class weekday;

constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;

constexpr weekday operator+(const weekday& x, const days&    y) noexcept;
constexpr weekday operator+(const days&    x, const weekday& y) noexcept;
constexpr weekday operator-(const weekday& x, const days&    y) noexcept;
constexpr days    operator-(const weekday& x, const weekday& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday& wd);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const weekday& wd);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class weekday_indexed;

constexpr bool operator==(const weekday_indexed& x, const weekday_indexed& y) noexcept;
constexpr bool operator!=(const weekday_indexed& x, const weekday_indexed& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday_indexed& wdi);

class weekday_last;

constexpr bool operator==(const weekday_last& x, const weekday_last& y) noexcept;
constexpr bool operator!=(const weekday_last& x, const weekday_last& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday_last& wdl);

class month_day;

constexpr bool operator==(const month_day& x, const month_day& y) noexcept;
constexpr bool operator!=(const month_day& x, const month_day& y) noexcept;
constexpr bool operator< (const month_day& x, const month_day& y) noexcept;
constexpr bool operator> (const month_day& x, const month_day& y) noexcept;
constexpr bool operator<=(const month_day& x, const month_day& y) noexcept;
constexpr bool operator>=(const month_day& x, const month_day& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_day& md);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month_day& md);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                month_day& md, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class month_day_last;

constexpr bool operator==(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator!=(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator< (const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator> (const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator<=(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator>=(const month_day_last& x, const month_day_last& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_day_last& mdl);

class month_weekday;

constexpr bool operator==(const month_weekday& x, const month_weekday& y) noexcept;
constexpr bool operator!=(const month_weekday& x, const month_weekday& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_weekday& mwd);

class month_weekday_last;

constexpr bool operator==(const month_weekday_last& x, const month_weekday_last& y) noexcept;
constexpr bool operator!=(const month_weekday_last& x, const month_weekday_last& y) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_weekday_last& mwdl);

class year_month;

constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
constexpr bool operator!=(const year_month& x, const year_month& y) noexcept;
constexpr bool operator< (const year_month& x, const year_month& y) noexcept;
constexpr bool operator> (const year_month& x, const year_month& y) noexcept;
constexpr bool operator<=(const year_month& x, const year_month& y) noexcept;
constexpr bool operator>=(const year_month& x, const year_month& y) noexcept;

constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
constexpr months operator-(const year_month& x, const year_month& y) noexcept;
constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month& ym);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month& ym);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class year_month_day;

constexpr bool operator==(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator!=(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator< (const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator> (const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator<=(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator>=(const year_month_day& x, const year_month_day& y) noexcept;

constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;
constexpr year_month_day operator+(const years& dy, const year_month_day& ymd) noexcept;
constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;
constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_day& ymd);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month_day& ymd);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year_month_day& ymd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);

class year_month_day_last;

constexpr bool operator==(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator!=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator< (const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator> (const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator<=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator>=(const year_month_day_last& x, const year_month_day_last& y) noexcept;

constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const months& dm) noexcept;
constexpr year_month_day_last operator+(const months& dm, const year_month_day_last& ymdl) noexcept;
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const years& dy) noexcept;
constexpr year_month_day_last operator+(const years& dy, const year_month_day_last& ymdl) noexcept;
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const months& dm) noexcept;
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const years& dy) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_day_last& ymdl);

class year_month_weekday;

constexpr bool operator==(const year_month_weekday& x, const year_month_weekday& y) noexcept;
constexpr bool operator!=(const year_month_weekday& x, const year_month_weekday& y) noexcept;

constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;
constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const years& dy) noexcept;
constexpr year_month_weekday operator+(const years& dy, const year_month_weekday& ymwd) noexcept;
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const years& dy) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_weekday& ymwdi);

class year_month_weekday_last;

constexpr bool operator==(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;
constexpr bool operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;

constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
constexpr year_month_weekday_last operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;
constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const years& dy) noexcept;
constexpr year_month_weekday_last operator+(const years& dy, const year_month_weekday_last& ymwdl) noexcept;
constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const years& dy) noexcept;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_weekday_last& ymwdl);

// civil calendar conventional syntax operators
constexpr year_month operator/(const year& y, const month& m) noexcept;
constexpr year_month operator/(const year& y, int   m) noexcept;
constexpr month_day operator/(const month& m, const day& d) noexcept;
constexpr month_day operator/(const month& m, int d) noexcept;
constexpr month_day operator/(int m, const day& d) noexcept;
constexpr month_day operator/(const day& d, const month& m) noexcept;
constexpr month_day operator/(const day& d, int m) noexcept;
constexpr month_day_last operator/(const month& m, last_spec) noexcept;
constexpr month_day_last operator/(int m, last_spec) noexcept;
constexpr month_day_last operator/(last_spec, const month& m) noexcept;
constexpr month_day_last operator/(last_spec, int m) noexcept;
constexpr month_weekday operator/(const month& m, const weekday_indexed& wdi) noexcept;
constexpr month_weekday operator/(int m, const weekday_indexed& wdi) noexcept;
constexpr month_weekday operator/(const weekday_indexed& wdi, const month& m) noexcept;
constexpr month_weekday operator/(const weekday_indexed& wdi, int m) noexcept;
constexpr month_weekday_last operator/(const month& m, const weekday_last& wdl) noexcept;
constexpr month_weekday_last operator/(int m, const weekday_last& wdl) noexcept;
constexpr month_weekday_last operator/(const weekday_last& wdl, const month& m) noexcept;
constexpr month_weekday_last operator/(const weekday_last& wdl, int m) noexcept;
constexpr year_month_day operator/(const year_month& ym, const day& d) noexcept;
constexpr year_month_day operator/(const year_month& ym, int d) noexcept;
constexpr year_month_day operator/(const year& y, const month_day& md) noexcept;
constexpr year_month_day operator/(int y, const month_day& md) noexcept;
constexpr year_month_day operator/(const month_day& md, const year& y) noexcept;
constexpr year_month_day operator/(const month_day& md, int y) noexcept;
constexpr year_month_day_last operator/(const year_month& ym, last_spec) noexcept;
constexpr year_month_day_last operator/(const year& y, const month_day_last& mdl) noexcept;
constexpr year_month_day_last operator/(int y, const month_day_last& mdl) noexcept;
constexpr year_month_day_last operator/(const month_day_last& mdl, const year& y) noexcept;
constexpr year_month_day_last operator/(const month_day_last& mdl, int y) noexcept;
constexpr year_month_weekday operator/(const year_month& ym, const weekday_indexed& wdi) noexcept;
constexpr year_month_weekday operator/(const year& y, const month_weekday& mwd) noexcept;
constexpr year_month_weekday operator/(int y, const month_weekday& mwd) noexcept;
constexpr year_month_weekday operator/(const month_weekday& mwd, const year& y) noexcept;
constexpr year_month_weekday operator/(const month_weekday& mwd, int y) noexcept;
constexpr year_month_weekday_last operator/(const year_month& ym, const weekday_last& wdl) noexcept;
constexpr year_month_weekday_last operator/(const year& y, const month_weekday_last& mwdl) noexcept;
constexpr year_month_weekday_last operator/(int y, const month_weekday_last& mwdl) noexcept;
constexpr year_month_weekday_last operator/(const month_weekday_last& mwdl, const year& y) noexcept;
constexpr year_month_weekday_last operator/(const month_weekday_last& mwdl, int y) noexcept;

// time_of_day
template <class Duration> class time_of_day;
template <> class time_of_day<hours>;
template <> class time_of_day<minutes>;
template <> class time_of_day<seconds>;
template <class Rep, class Period> class time_of_day<duration<Rep, Period>>;

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const time_of_day<hours>& t);

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const time_of_day<minutes>& t);

template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const time_of_day<seconds>& t);

template<class charT, class traits, class Rep, class Period>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const time_of_day<duration<Rep, Period>>& t);

// time zone database

struct tzdb;
class tzdb_list;
const tzdb& get_tzdb();
tzdb_list& get_tzdb_list();
const time_zone* locate_zone(string_view tz_name);
const time_zone* current_zone();

// Remote time zone database -- Needs discussion

const tzdb&  reload_tzdb();
string       remote_version();
bool         remote_download(string_view version);
bool         remote_install(string_view version);

// exception classes
class nonexistent_local_time;
class ambiguous_local_time;

// information classes
struct sys_info;
template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const sys_info& si);

struct local_info;
template<class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const local_info& li);

// time_zone
enum class choose {earliest, latest};
class time_zone;

bool operator==(const time_zone& x, const time_zone& y) noexcept;
bool operator!=(const time_zone& x, const time_zone& y) noexcept;

bool operator<(const time_zone& x, const time_zone& y) noexcept;
bool operator>(const time_zone& x, const time_zone& y) noexcept;
bool operator<=(const time_zone& x, const time_zone& y) noexcept;
bool operator>=(const time_zone& x, const time_zone& y) noexcept;

// zoned_time
template <class Duration, class TimeZonePtr = const time_zone*> class zoned_time;

using zoned_seconds = zoned_time<seconds>;

template <class Duration1, class Duration2, class TimeZonePtr>
    bool
    operator==(const zoned_time<Duration1, TimeZonePtr>& x,
               const zoned_time<Duration2, TimeZonePtr>& y);

template <class Duration1, class Duration, class TimeZonePtr2>
    bool
    operator!=(const zoned_time<Duration1, TimeZonePtr>& x,
               const zoned_time<Duration2, TimeZonePtr>& y);

template <class charT, class traits, class Duration, class TimeZonePtr>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               const zoned_time<Duration, TimeZonePtr>& t);

template <class charT, class traits, class Duration, class TimeZonePtr>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt,
              const zoned_time<Duration, TimeZonePtr>& tp);

// format

template <class charT, class Streamable>
    basic_string<charT>
    format(const charT* fmt, const Streamable& s);

template <class charT, class Streamable>
    basic_string<charT>
    format(const locale& loc, const charT* fmt, const Streamable& s);

template <class charT, class traits, class Alloc, class Streamable>
    basic_string<charT, traits, Alloc>
    format(const basic_string<charT, traits, Alloc>& fmt, const Streamable& s);

template <class charT, class traits, class Alloc, class Streamable>
    basic_string<charT, traits, Alloc>
    format(const locale& loc, const basic_string<charT, traits, Alloc>& fmt, const Streamable& s);

// parse

template <class charT, class traits, class Alloc, class Parsable>
    unspecified
    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);

template <class charT, class traits, class Alloc, class Parsable>
    unspecified
    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
          basic_string<charT, traits, Alloc>& abbrev);

template <class charT, class traits, class Alloc, class Parsable>
    unspecified
    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
          minutes& offset);

template <class charT, class traits, class Alloc, class Parsable>
    unspecified
    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
          basic_string<charT, traits, Alloc>& abbrev, minutes& offset);

// leap second support

class leap;

bool operator==(const leap& x, const leap& y);
bool operator!=(const leap& x, const leap& y);
bool operator< (const leap& x, const leap& y);
bool operator> (const leap& x, const leap& y);
bool operator<=(const leap& x, const leap& y);
bool operator>=(const leap& x, const leap& y);

template <class Duration> bool operator==(const const leap&         x, const sys_time<Duration>& y);
template <class Duration> bool operator==(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator!=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator!=(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator< (const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator< (const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator> (const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator> (const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator<=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator<=(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator>=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator>=(const sys_time<Duration>& x, const leap&               y);

class link;

bool operator==(const link& x, const link& y);
bool operator!=(const link& x, const link& y);
bool operator< (const link& x, const link& y);
bool operator> (const link& x, const link& y);
bool operator<=(const link& x, const link& y);
bool operator>=(const link& x, const link& y);

} // namespace chrono

inline namespace literals {
inline namespace chrono_literals {
// ...
inline constexpr chrono::last_spec last{};

inline constexpr chrono::weekday sun{0};
inline constexpr chrono::weekday mon{1};
inline constexpr chrono::weekday tue{2};
inline constexpr chrono::weekday wed{3};
inline constexpr chrono::weekday thu{4};
inline constexpr chrono::weekday fri{5};
inline constexpr chrono::weekday sat{6};

inline constexpr chrono::month jan{1};
inline constexpr chrono::month feb{2};
inline constexpr chrono::month mar{3};
inline constexpr chrono::month apr{4};
inline constexpr chrono::month may{5};
inline constexpr chrono::month jun{6};
inline constexpr chrono::month jul{7};
inline constexpr chrono::month aug{8};
inline constexpr chrono::month sep{9};
inline constexpr chrono::month oct{10};
inline constexpr chrono::month nov{11};
inline constexpr chrono::month dec{12};

constexpr chrono::day  operator "" d(unsigned long long d) noexcept;
constexpr chrono::year operator "" y(unsigned long long y) noexcept;
}
}

Back to TOC

Add new section [time.is_clock] after 23.17.4.3 Specializations of common_type [time.traits.specializations]:

23.17.4.4 is_clock [time.traits.is_clock]
template <class T> struct is_clock;
is_clock is a UnaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if T meets the Clock requirements ([time.clock.req]), otherwise false_type. For the purposes of the specification of this trait, the extent to which an implementation determines that a type cannot meet the clock requirements is unspecified, except that as a minimum a type T shall not qualify as a clock unless it satisfies all of the following conditions:

the qualified-ids T::rep, T::period, T::duration, and T::time_point are valid and each denotes a type ([temp.deduct]),

the expression T::is_steady is well-formed when treated as an unevaluated operand,

the expression T::now() is well-formed when treated as an unevaluated operand.

The behavior of a program that adds specializations for is_clock is undefined.

Back to TOC

Add new section [time.duration.io] after 23.17.5.9 duration algorithms [time.duration.alg]:

23.17.5.10 duration stream insertion [time.duration.io]
template <class charT, class traits, class Rep, class Period>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);
Effects: Forms a basic_string<charT, traits> from d.count() appended with get_units<charT, traits>(typename Period::type{}) (described below) and inserts that basic_string into os. [Note: this specification assures that the result of this streaming operation will obey the width and alignment properties of the stream. — end note]

get_units<charT, traits>(typename Period::type{}) is an exposition-only function which returns a null-terminated string of charT which depends on Period::type as follows (let period be the type Period::type):

If period is type atto, as, else

if period is type femto, fs, else

if period is type pico, ps, else

if period is type nano, ns, else

if period is type micro, µs (U+00B5), else

if period is type milli, ms, else

if period is type centi, cs, else

if period is type deci, ds, else

if period is type ratio<1>, s, else

if period is type deca, das, else

if period is type hecto, hs, else

if period is type kilo, ks, else

if period is type mega, Ms, else

if period is type giga, Gs, else

if period is type tera, Ts, else

if period is type peta, Ps, else

if period is type exa, Es, else

if period is type ratio<60>, min, else

if period is type ratio<3600>, h, else

if period::den == 1, [num]s, else

[num/den]s.

In the list above the use of num and den refer to the static data members of period which are converted to arrays of charT using a decimal conversion with no leading zeroes.

For streams with charT which has a representation of 8 bits µs should be encoded as UTF-8. Otherwise UTF-16 or UTF-32 is encouraged. The implementation may substitute other encodings, including us.

Returns: os.
template <class charT, class traits, class Rep, class Period>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt,
          const duration<Rep, Period>& d);
Effects: Streams d into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            duration<Rep, Period>& d,
            basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the duration d using the format flags as specified in [time.parse].

If the parse parses everything specified by the parsing format flags without error, and yet none of the flags impacts a duration, d will be assigned a zero value.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

Back to TOC

Add to synopsis in section [time.point] 23.17.6 Class template time_point:

template <class Clock, class Duration = typename Clock::duration>
class time_point {
public:
...
// 23.17.6.3, arithmetic
constexpr time_point& operator++();
constexpr time_point operator++(int);
constexpr time_point& operator--();
constexpr time_point operator--(int);

constexpr time_point& operator+=(const duration& d);
constexpr time_point& operator-=(const duration& d);
...
};

Back to TOC

Modify section 23.17.6 Class template time_point [time.point]/p1:

1 Clock shall meet the Clock requirements ([time.clock.req]) or Clock shall be local_t.

Back to TOC

Add to section [time.point.arithmetic] 23.17.6.3 time_point arithmetic:

constexpr time_point& operator++();
Effects: ++d_.

Returns: *this.
constexpr time_point operator++(int);
Returns: time_point{d_++}.
constexpr time_point& operator--();
Effects: --d_.

Returns: *this.
constexpr time_point operator--(int);
Returns: time_point{d_--}.

Back to TOC

Modify 23.17.7 [time.clock]:

1 The types defined in this subclause shall satisfy the TrivialClock requirements (23.17.3) unless otherwise specified.

Back to TOC

Modify 23.17.7.1 [time.clock.system]:

1 Objects of class system_clock represent wall clock time from the system-wide realtime clock. sys_time<Duration> measures time since (and before) 1970-01-01 00:00:00 UTC excluding leap seconds. This measure is commonly referred to as Unix Time. This measure facilitates an efficient mapping between sys_time and calendar types ([time.calendar])

[Example:

sys_seconds{sys_days{1970y/jan/1}}.time_since_epoch() is 0s sys_seconds{sys_days{2000y/jan/1}}.time_since_epoch() is 946'684'800s which is 10'957 * 86'400s

—end example]

Back to TOC

Deprecate the to_time_t and from_time_t static member functions of system_clock in 23.17.7.1 [time.clock.system] by moving their declaration and specification to Annex D.

Rationale: This proposal removes all need for using the C <time.h> API except to translate system_clock::time_point to legacy code. That translation can now be portably done by converting a system_clock::time_point to a year_month_day and time_of_day<seconds> and translating that information into a tm. The C API is error prone with its lack of type safe distinctions between local time and UTC, and suffers from a lack of handling precisions finer than seconds. Furthermore the newer timespec API makes no distinction between time points and time durations, further eroding type safety.

Back to TOC

Append new paragraphs after 23.17.7.1 [time.clock.system]/p4:

template <class Duration>
sys_time<common_type_t<Duration, seconds>>
to_sys_time(const utc_time<Duration>& u);
Returns: A sys_time t, such that to_utc_time(t) == u if such a mapping exists. Otherwise u represents a time_point during a leap second insertion and the last representable value of sys_time prior to the insertion of the leap second is returned.

[Example:
auto t = sys_days{jul/1/2015} - 500ms;
auto u = to_utc_time(t);
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25001ms);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25251ms);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25501ms);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 25751ms);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s);
cout << t << " SYS == " << u << " UTC\n";
u += 250ms;
t = to_sys_time(u);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s);
cout << t << " SYS == " << u << " UTC\n";
Output:
2015-06-30 23:59:59.500 SYS == 2015-06-30 23:59:59.500 UTC
2015-06-30 23:59:59.750 SYS == 2015-06-30 23:59:59.750 UTC
2015-06-30 23:59:59.999 SYS == 2015-06-30 23:59:60.000 UTC
2015-06-30 23:59:59.999 SYS == 2015-06-30 23:59:60.250 UTC
2015-06-30 23:59:59.999 SYS == 2015-06-30 23:59:60.500 UTC
2015-06-30 23:59:59.999 SYS == 2015-06-30 23:59:60.750 UTC
2015-07-01 00:00:00.000 SYS == 2015-07-01 00:00:00.000 UTC
2015-07-01 00:00:00.250 SYS == 2015-07-01 00:00:00.250 UTC
— end example]
template <class Duration>
sys_time<common_type_t<Duration, seconds>>
to_sys_time(const tai_time<Duration>& u);
Effects: Equivalent to: return to_sys_time(to_utc_time(u));
template <class Duration>
sys_time<common_type_t<Duration, seconds>>
to_sys_time(const gps_time<Duration>& u);
Effects: Equivalent to: return to_sys_time(to_utc_time(u));
template <class Duration>
sys_time<Duration>
to_sys_time(const file_time<Duration>& u);
Returns: A sys_time t, such that to_file_time(t) == u if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);
Remarks: This operator shall not participate in overload resolution if treat_as_floating_point<typename Duration::rep>::value is true, or if Duration{1} >= days{1}.

Effects:
auto const dp = floor<days>(tp);
os << year_month_day{dp} << ' ' << time_of_day{tp-dp};
Returns: os.

[Example:
cout << sys_seconds{0s} << '\n';             // 1970-01-01 00:00:00
cout << sys_seconds{946'684'800s} << '\n';   // 2000-01-01 00:00:00
— end example:]
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const sys_days& dp);
Effects:
os << year_month_day{dp};
Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const sys_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with "UTC". If %z is used (or a modified form of %z), an offset of 0min will be formatted.

Returns: os.
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            sys_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the sys_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null. Additionally, the parsed offset will be subtracted from the successfully parsed time stamp prior to assigning that difference to tp.

Returns: is.

Back to TOC

Add new section [time.clock.utc] after 23.17.7.1 Class system_clock [time.clock.system]:

23.17.7.2 Class utc_clock [time.clock.utc]
class utc_clock
{
public:
    using duration                  = system_clock::duration;
    using rep                       = duration::rep;
    using period                    = duration::period;
    using time_point                = chrono::time_point<utc_clock>;
    static constexpr bool is_steady = unspecified;

    static time_point now();
};
In contrast to sys_time which does not take leap seconds into account, utc_clock and its associated time_point, utc_time, counts time, including leap seconds, since 1970-01-01 00:00:00 UTC.

[Example:
to_utc_time(sys_seconds{sys_days{1970y/jan/1}}).time_since_epoch() is 0s
to_utc_time(sys_seconds{sys_days{2000y/jan/1}}).time_since_epoch() is 946'684'822s which is 10'957 * 86'400s + 22s
—end example]

utc_clock is not a TrivialClock unless the implementation can guarantee that utc_clock::now() does not propagate an exception. [Note: noexcept(to_utc_time(system_clock::now())) is false. — end note]
static utc_clock::time_point utc_clock::now();
Returns: The implementations should supply the best measure available. This may be approximated with to_utc_time(system_clock::now()).
template <class Duration>
utc_time<common_type_t<Duration, seconds>>
to_utc_time(const sys_time<Duration>& t);
Returns: A utc_time u, such that u.time_since_epoch() - t.time_since_epoch() is equal to the number of leap seconds that were inserted between t and 1970-01-01. If t is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.

[Example:
auto t = sys_days{jul/1/2015} - 2ns;
auto u = to_utc_time(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
t += 1ns;
u = to_utc_time(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 25s);
t += 1ns;
u = to_utc_time(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s);
t += 1ns;
u = to_utc_time(t);
assert(u.time_since_epoch() - t.time_since_epoch() == 26s);
— end example]
template <class Duration>
utc_time<common_type_t<Duration, seconds>>
to_utc_time(const tai_time<Duration>& t);
Returns: utc_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 378691210s

Note: 378691210s == sys_days{1970y/jan/1} - sys_days{1958y/jan/1} + 10s
template <class Duration>
utc_time<common_type_t<Duration, seconds>>
to_utc_time(const gps_time<Duration>& t);
Returns: utc_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 315964809s

Note: 315964809s == sys_days{1980y/jan/sun[1]} - sys_days{1970y/jan/1} + 9s
template <class Duration>
utc_time<common_type_t<Duration, seconds>>
to_utc_time(const file_time<Duration>& t);
Returns: A utc_time t, such that to_file_time(t) == u if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const utc_time<Duration>& t);
Effects: Calls to_stream(os, fmt, t), where fmt is a null-terminated array of char_t containing "%F %T" widened for charT.

Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const utc_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with "UTC". If %z is used (or a modified form of %z), an offset of 0min will be formatted. If tp represents a time during a leap second insertion, and if a seconds field is formatted, the integral portion of that format shall be "60".

Returns: os.

[Example:
auto t = sys_days{jul/1/2015} - 500ms;
auto u = to_utc_time(t);
for (auto i = 0; i < 8; ++i, u += 250ms)
    cout << u << " UTC\n";
Output:
2015-06-30 23:59:59.500 UTC
2015-06-30 23:59:59.750 UTC
2015-06-30 23:59:60.000 UTC
2015-06-30 23:59:60.250 UTC
2015-06-30 23:59:60.500 UTC
2015-06-30 23:59:60.750 UTC
2015-07-01 00:00:00.000 UTC
2015-07-01 00:00:00.250 UTC
— end example]
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            utc_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the utc_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null. Additionally, the parsed offset will be subtracted from the successfully parsed time stamp prior to assigning that difference to tp.

Returns: is.

Back to TOC

Add new section [time.clock.tai] after 23.17.7.2 Class utc_clock [time.clock.utc]:

23.17.7.3 Class tai_clock [time.clock.tai]
class tai_clock
{
public:
    using duration                  = system_clock::duration;
    using rep                       = duration::rep;
    using period                    = duration::period;
    using time_point                = chrono::time_point<tai_clock>;
    static constexpr bool is_steady = unspecified;

    static time_point now();
};
The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into TAI. Therefore every time a leap second is inserted into UTC, UTC falls another second behind TAI. For example by 2000-01-01 there had been 22 leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).

tai_clock is not a TrivialClock unless the implementation can guarantee that tai_clock::now() does not propagate an exception. [Note: noexcept(to_tai_time(system_clock::now())) is false. — end note]
static tai_clock::time_point tai_clock::now();
Returns: The implementations should supply the best measure available. This may be approximated with to_tai_time(system_clock::now()).
template <class Duration>
tai_time<common_type_t<Duration, seconds>>
to_tai_time(const sys_time<Duration>& t);
Effects: Equivalent to: return to_tai_time(to_utc_time(t));.
template <class Duration>
tai_time<common_type_t<Duration, seconds>>
to_tai_time(const utc_time<Duration>& t);
Returns: tai_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 378691210s

Note: 378691210s == sys_days{1970y/jan/1} - sys_days{1958y/jan/1} + 10s
template <class Duration>
tai_time<common_type_t<Duration, seconds>>
to_tai_time(const gps_time<Duration>& t);
Returns: tai_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 694656019s

Note: 694656019s == sys_days{1980y/jan/sun[1]} - sys_days{1958y/jan/1} + 19s
template <class Duration>
tai_time<common_type_t<Duration, seconds>>
to_tai_time(const file_time<Duration>& t);
Returns: A tai_time t, such that to_file_time(t) == u if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const tai_time<Duration>& t);
Effects: Calls to_stream(os, fmt, t), where fmt is a null-terminated array of char_t containing "%F %T" widened for charT.

Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const tai_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with "TAI". If %z is used (or a modified form of %z), an offset of 0min will be formatted. The date and time formatted shall be equivalent to that formatted by a sys_time initialized with:
sys_time<Duration>{tp.time_since_epoch()} - (sys_days{1970y/jan/1} - sys_days{1958y/jan/1})
Returns: os.

[Example:
auto st = sys_days{2000_y/jan/1};
auto tt = to_tai_time(st);
cout << format("%F %T %Z == ", st) << format("%F %T %Z\n", tt);
Output:
2000-01-01 00:00:00 UTC == 2000-01-01 00:00:32 TAI
— end example]
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            tai_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the tai_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null. Additionally, the parsed offset will be subtracted from the successfully parsed time stamp prior to assigning that difference to tp.

Returns: is.

Back to TOC

Add new section [time.clock.gps] after 23.17.7.3 Class tai_clock [time.clock.tai]:

23.17.7.4 Class gps_clock [time.clock.gps]
class gps_clock
{
public:
    using duration                  = system_clock::duration;
    using rep                       = duration::rep;
    using period                    = duration::period;
    using time_point                = chrono::time_point<gps_clock>;
    static constexpr bool is_steady = unspecified;

    static time_point now();
};
The clock gps_clock measures seconds since The first Sunday of January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS. Therefore every time a leap second is inserted into UTC, UTC falls another second behind GPS. Aside from the offset from 1958y/jan/1 to 1980y/jan/sun[1] GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.

gps_clock is not a TrivialClock unless the implementation can guarantee that gps_clock::now() does not propagate an exception. [Note: noexcept(to_gps_time(system_clock::now())) is false. — end note]
static gps_clock::time_point gps_clock::now();
Returns: The implementations should supply the best measure available. This may be approximated with to_gps_time(system_clock::now()).
template <class Duration>
gps_time<common_type_t<Duration, seconds>>
to_gps_time(const sys_time<Duration>& t);
Effects: Equivalent to: return to_gps_time(to_utc_time(t));.
template <class Duration>
gps_time<common_type_t<Duration, seconds>>
to_gps_time(const utc_time<Duration>& t);
Returns: gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 315964809s

Note: 315964809s == sys_days{1980y/jan/sun[1]} - sys_days{1970y/jan/1} + 9s
template <class Duration>
gps_time<common_type_t<Duration, seconds>>
to_gps_time(const tai_time<Duration>& t);
Returns: gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 694656019s

Note: 694656019s == sys_days{1980y/jan/sun[1]} - sys_days{1958y/jan/1} + 19s
template <class Duration>
gps_time<common_type_t<Duration, seconds>>
to_gps_time(const file_time<Duration>& t);
Returns: A gps_time t, such that to_file_time(t) == u if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const gps_time<Duration>& t);
Effects: Calls to_stream(os, fmt, t), where fmt is a null-terminated array of char_t containing "%F %T" widened for charT.

Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const gps_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with "GPS". If %z is used (or a modified form of %z), an offset of 0min will be formatted. The date and time formatted shall be equivalent to that formatted by a sys_time initialized with:
sys_time<Duration>{tp.time_since_epoch()} + (sys_days{1980y/jan/sun[1]} - sys_days{1970y/jan/1})
Returns: os.

[Example:
auto st = sys_days{2000_y/jan/1};
auto gt = to_gps_time(st);
cout << format("%F %T %Z == ", st) << format("%F %T %Z\n", gt);
Output:
2000-01-01 00:00:00 UTC == 2000-01-01 00:00:13 GPS
— end example]
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            gps_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the gps_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null. Additionally, the parsed offset will be subtracted from the successfully parsed time stamp prior to assigning that difference to tp.

Returns: is.

Back to TOC

Add new section [time.clock.file] after 23.17.7.4 Class gps_clock [time.clock.gps]:

23.17.7.5 Class file_clock [time.clock.file]
class file_clock
{
public:
    using rep                       = unspecified;
    using period                    = ratio<unspecified, unspecified>;
    using duration                  = chrono::duration<rep, period>;
    using time_point                = chrono::time_point<file_clock>;
    static constexpr bool is_steady = unspecified;

    static time_point now() noexcept;
};
The clock file_clock is used to create the time_point system used for file_time_type ([filesystems]). It's epoch is unspecified.
template <class Duration>
file_time<Duration>
to_file_time(const sys_time<Duration>& t);
Returns: A file_time u, such that to_sys_time(u) == t if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class Duration>
file_time<common_type_t<Duration, seconds>>
to_file_time(const utc_time<Duration>& t);
Returns: A file_time u, such that to_utc_time(u) == t if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class Duration>
file_time<common_type_t<Duration, seconds>>
to_file_time(const tai_time<Duration>& t);
Returns: A file_time u, such that to_tai_time(u) == t if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class Duration>
file_time<common_type_t<Duration, seconds>>
to_file_time(const gps_time<Duration>& t);
Returns: A file_time t, such that to_gps_time(t) == u if such a mapping exists. t and u should represent the same point in time, even though they may have different epochs.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& t);
Effects: Calls to_stream(os, fmt, t), where fmt is a null-terminated array of char_t containing "%F %T" widened for charT.

Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const file_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with "UTC". If %z is used (or a modified form of %z), an offset of 0min will be formatted. The date and time formatted shall be equivalent to that formatted by a sys_time initialized with to_sys_time(tp).

Returns: os.
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            file_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the file_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null. Additionally, the parsed offset will be subtracted from the successfully parsed time stamp prior to assigning that difference to tp.

Returns: is.

Back to TOC

Add new section [time.clock.local_time] after 23.17.7.3 Class high_resolution_clock [time.clock.hres]:

23.17.7.8 local_time [time.clock.local_time]

The family of time points denoted by local_time<Duration> are based on the pseudo clock local_t. local_t has no member now() and thus does not meet the clock requirements. Nevertheless local_time<Duration> serves the vital role of representing local time with respect to a not-yet-specified time zone. Aside from being able to get the current time, the complete time_point algebra is available for local_time<Duration> (just as for sys_time<Duration>).
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const local_time<Duration>& lt);
Effects:
os << sys_time<Duration>{lt.time_since_epoch()};
Returns: os.
template <class charT, class traits, class Duration>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const local_time<Duration>& tp,
          const string* abbrev = nullptr, const seconds* offset_sec = nullptr);
Effects: Streams tp into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format]. If %Z is used, it will be replaced with *abbrev if abbrev is not equal to nullptr. If abbrev is equal to nullptr (and %Z is used), ios_base::failbit will be set. If %z is used (or a modified form of %z), it will be formatted with the value of *offset_sec if offset_sec is not equal to nullptr. If %z (or a modified form of %z) is used, and offset_sec is equal to nullptr, then ios_base::failbit is set.

Returns: os.
template <class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            local_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the local_time tp using the format flags as specified in [time.parse].

If the parse fails to decode a valid date, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

Back to TOC

Add a new section 23.17.8 Formatting [time.format]:

If fmt (P0645) moves forward within the LEWG, this section can easily be reworked to plug into that facility without loss of functionality. This will avoid two unrelated format facilities in the standard.

23.17.8 Formatting [time.format]

Each format overload specified in this section calls to_stream unqualified, so as to enable argument dependent lookup ([basic.lookup.argdep]).
template <class charT, class Streamable>
basic_string<charT>
format(const charT* fmt, const Streamable& s);
Remarks: This function shall not participate in overload resolution unless to_stream(declval<basic_ostream<charT>&>(), fmt, s) is a valid expression.

Effects: Constructs a local variable of type basic_ostringstream<charT> (for exposition purposes, named os). Executes os.exceptions(ios::failbit | ios::badbit). Then calls to_stream(os, fmt, s).

Returns: os.str().
template <class charT, class Streamable>
basic_string<charT>
format(const locale& loc, const charT* fmt, const Streamable& s);
Remarks: This function shall not participate in overload resolution unless to_stream(declval<basic_ostream<charT>&>(), fmt, s) is a valid expression.

Effects: Constructs a local variable of type basic_ostringstream<charT> (for exposition purposes, named os). Executes os.exceptions(ios::failbit | ios::badbit). Then calls os.imbue(loc). Then calls to_stream(os, fmt, s).

Returns: os.str().
template <class charT, class traits, class Alloc, class Streamable>
basic_string<charT, traits, Alloc>
format(const basic_string<charT, traits, Alloc>& fmt, const Streamable& s);
Remarks: This function shall not participate in overload resolution unless to_stream(declval<basic_ostringstream<charT, traits, Alloc>&>(), fmt.c_str(), s) is a valid expression.

Effects: Constructs a local variable of type basic_ostringstream<charT, traits, Alloc> (for exposition purposes, named os). Executes os.exceptions(ios::failbit | ios::badbit). Then calls to_stream(os, fmt.c_str(), s).

Returns: os.str().
template <class charT, class traits, class Alloc, class Streamable>
basic_string<charT, traits, Alloc>
format(const locale& loc, const basic_string<charT, traits, Alloc>& fmt, const Streamable& s);
Remarks: This function shall not participate in overload resolution unless to_stream(declval<basic_ostringstream<charT, traits, Alloc>&>(), fmt.c_str(), s) is a valid expression.

Effects: Constructs a local variable of type basic_ostringstream<charT, traits, Alloc> (for exposition purposes, named os). Then calls os.imbue(loc). Executes os.exceptions(ios::failbit | ios::badbit). Then calls to_stream(os, fmt.c_str(), s).

Returns: os.str().

The format functions call a to_stream function with a basic_ostream, a formatting string specifier, and a Streamable argument. Each to_stream overload is customized for each Streamable type. However all to_stream overloads treat the formatting string specifier according to the following specification:

The fmt string consists of zero or more conversion specifiers and ordinary multibyte characters. A conversion specifier consists of a % character, possibly followed by an E or O modifier character (described below), followed by a character that determines the behavior of the conversion specifier. All ordinary multibyte characters (excluding the terminating null character) are streamed unchanged into the basic_ostream.

Each conversion specifier is replaced by appropriate characters as described in the following list. Some of the conversion specifiers depend on the locale which is imbued to the basic_ostream. If the Streamable object does not contain the information the conversion specifier refers to, the value streamed to the basic_ostream is unspecified.

Unless explicitly specified, Streamable types will not contain time zone abbreviation and time zone offset information. If available, the conversion specifiers %Z and %z will format this information (respectively). If the information is not available, and %Z or %z are contained in fmt, ios_base::failbit will be set on os.

%a The locale's abbreviated weekday name. If the value does not contain a valid weekday, ios::failbit is set.

%A The locale's full weekday name. If the value does not contain a valid weekday, ios::failbit is set.

%b The locale's abbreviated month name. If the value does not contain a valid month, ios::failbit is set.

%B The locale's full month name. If the value does not contain a valid month, ios::failbit is set.

%c The locale's date and time representation. The modified command %Ec produces the locale's alternate date and time representation.

%C The year divided by 100 using floored division. If the result is a single decimal digit, it is prefixed with 0. The modified command %EC produces the locale's alternative representation of the century.

%d The day of month as a decimal number. If the result is a single decimal digit, it is prefixed with 0. The modified command %Od produces the locale's alternative representation.

%D Equivalent to %m/%d/%y.

%e The day of month as a decimal number. If the result is a single decimal digit, it is prefixed with a space. The modified command %Oe produces the locale's alternative representation.

%F Equivalent to %Y-%m-%d.

%g The last two decimal digits of the ISO week-based year. If the result is a single digit it is prefixed by 0.

%G The ISO week-based year as a decimal number. If the result is less than four digits it is left-padded with 0 to four digits.

%h Equivalent to %b.

%H The hour (24-hour clock) as a decimal number. If the result is a single digit, it is prefixed with 0. The modified command %OH produces the locale's alternative representation.

%I The hour (12-hour clock) as a decimal number. If the result is a single digit, it is prefixed with 0. The modified command %OI produces the locale's alternative representation.

%j The day of the year as a decimal number. Jan 1 is 001. If the result is less than three digits, it is left-padded with 0 to three digits.

%m The month as a decimal number. Jan is 01. If the result is a single digit, it is prefixed with 0. The modified command %Om produces the locale's alternative representation.

%M The minute as a decimal number. If the result is a single digit, it is prefixed with 0. The modified command %OM produces the locale's alternative representation.

%n A newline character.

%p The locale's equivalent of the AM/PM designations associated with a 12-hour clock.

%r The locale's 12-hour clock time.

%R Equivalent to %H:%M.

%S Seconds as a decimal number. If the number of seconds is less than 10, the result is prefixed with 0. If the precision of the input can not be exactly represented with seconds, then the format is a decimal floating point number with a fixed format and a precision matching that of the precision of the input (or to a microseconds precision if the conversion to floating point decimal seconds can not be made within 18 fractional digits). The character for the decimal point is localized according to the locale. The modified command %OS produces the locale's alternative representation.

%t A horizontal-tab character.

%T Equivalent to %H:%M:%S.

%u The ISO weekday as a decimal number (1-7), where Monday is 1. The modified command %Ou produces the locale's alternative representation.

%U The week number of the year as a decimal number. The first Sunday of the year is the first day of week 01. Days of the same year prior to that are in week 00. If the result is a single digit, it is prefixed with 0. The modified command %OU produces the locale's alternative representation.

%V The ISO week-based week number as a decimal number. If the result is a single digit, it is prefixed with 0. The modified command %OV produces the locale's alternative representation.

%w The weekday as a decimal number (0-6), where Sunday is 0. The modified command %Ow produces the locale's alternative representation.

%W The week number of the year as a decimal number. The first Monday of the year is the first day of week 01. Days of the same year prior to that are in week 00. If the result is a single digit, it is prefixed with 0. The modified command %OW produces the locale's alternative representation.

%x The locale's date representation. The modified command %Ex produces the locale's alternate date representation.

%X The locale's time representation. The modified command %Ex produces the locale's alternate time representation.

%y The last two decimal digits of the year. If the result is a single digit it is prefixed by 0.

%Y The year as a decimal number. If the result is less than four digits it is left-padded with 0 to four digits.

%z The offset from UTC in the ISO 8601 format. For example -0430 refers to 4 hours 30 minutes behind UTC. If the offset is zero, +0000 is used. The modified commands %Ez and %Oz insert a : between the hours and minutes: -04:30. If the offset information is not available, ios_base::failbit will be set.

%Z The time zone abbreviation. If the time zone abbreviation is not available, ios_base::failbit will be set.

%% A % character.

Back to TOC

Add a new section 23.17.9 Parsing [time.parse]:

23.17.9 Parsing [time.parse]

Each parse overload specified in this section calls from_stream unqualified, so as to enable argument dependent lookup ([basic.lookup.argdep]).
template <class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);
Remarks: This function shall not participate in overload resolution unless from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp) is a valid expression.

Returns: A manipulator that when extracted from a basic_istream<charT, traits> is calls from_stream(is, fmt.c_str(), tp).
template <class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
      basic_string<charT, traits, Alloc>& abbrev);
Remarks: This function shall not participate in overload resolution unless from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, &abbrev) is a valid expression.

Returns: A manipulator that when extracted from a basic_istream<charT, traits> is calls from_stream(is, fmt.c_str(), tp, &abbrev).
template <class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
      minutes& offset);
Remarks: This function shall not participate in overload resolution unless from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, nullptr, &offset) is a valid expression.

Returns: A manipulator that when extracted from a basic_istream<charT, traits> is calls from_stream(is, fmt.c_str(), tp, nullptr, &offset).
template <class charT, class traits, class Alloc, class Parsable>
unspecified
parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp,
      basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
Remarks: This function shall not participate in overload resolution unless from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, &abbrev, &offset) is a valid expression.

Returns: A manipulator that when extracted from a basic_istream<charT, traits> is calls from_stream(is, fmt.c_str(), tp, &abbrev, &offset).

All from_stream overloads behave as an unformatted input function. Each overload takes a format string containing ordinary characters and flags which have special meaning. Each flag begins with a %. Some flags can be modified by E or O. During parsing each flag interprets characters as parts of date and time type according to the table below. Some flags can be modified by a width parameter which governs how many characters are parsed from the stream in interpreting the flag. All characters in the format string which are not represented in the table below, except for white space, are parsed unchanged from the stream. A white space character matches zero or more white space characters in the input stream.

If the from_stream overload fails to parse everything specified by the format string, or if insufficient information is parsed to specify a complete duration, time point, or calendrical data structure, ios_base::failbit is set in the basic_istream.

%a The locale's full or abbreviated case-insensitive weekday name.

%A Equivalent to %a.

%b The locale's full or abbreviated case-insensitive month name.

%B Equivalent to %b.

%c The locale's date and time representation. The modified command %Ec interprets the locale's alternate date and time representation.

%C The century as a decimal number. The modified command %NC where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands %EC and %OC interpret the locale's alternative representation of the century.

%d The day of the month as a decimal number. The modified command %Nd where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %EC interprets the locale's alternative representation of the day of the month.

%D Equivalent to %m/%d/%y.

%e Equivalent to %d and can be modified like %d.

%F Equivalent to %Y-%m-%d. If modified with a width, the width is applied to only %Y.

%g The last two decimal digits of the ISO week-based year. The modified command %Ng where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.

%G The ISO week-based year as a decimal number. The modified command %NG where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 4. Leading zeroes are permitted but not required.

%h Equivalent to %b.

%H The hour (24-hour clock) as a decimal number. The modified command %NH where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %OH interprets the locale's alternative representation.

%I The hour (12-hour clock) as a decimal number. The modified command %NI where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.

%j The day of the year as a decimal number. Jan 1 is 1. The modified command %Nj where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 3. Leading zeroes are permitted but not required.

%m The month as a decimal number. Jan is 1. The modified command %Nm where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %Om interprets the locale's alternative representation.

%M The minutes as a decimal number. The modified command %NM where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %OM interprets the locale's alternative representation.

%n Matches one white space character. [Note: %n, %t and a space, can be combined to match a wide range of white-space patterns. For example "%n " matches one or more white space characters, and "%n%t%t" matches one to three white space characters. — end note]

%p The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command %I must precede %p in the format string.

%r The locale's 12-hour clock time.

%R Equivalent to %H:%M.

%S The seconds as a decimal number. The modified command %NS where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2 if the input time has a precision convertible to seconds. Otherwise the default width is determined by the decimal precision of the input and the field is interpreted as a long double in a fixed format. If encountered, the locale determines the decimal point character. Leading zeroes are permitted but not required. The modified command %OS interprets the locale's alternative representation.

%t Matches zero or one white space characters.

%T Equivalent to %H:%M:%S.

%u The ISO weekday as a decimal number (1-7), where Monday is 1. The modified command %Nu where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 1. Leading zeroes are permitted but not required. The modified command %Ou interprets the locale's alternative representation.

%U The week number of the year as a decimal number. The first Sunday of the year is the first day of week 01. Days of the same year prior to that are in week 00. The modified command %NU where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.

%V The ISO week-based week number as a decimal number. The modified command %NV where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.

%w The weekday as a decimal number (0-6), where Sunday is 0. The modified command %Nw where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 1. Leading zeroes are permitted but not required. The modified command %Ow interprets the locale's alternative representation.

%W The week number of the year as a decimal number. The first Monday of the year is the first day of week 01. Days of the same year prior to that are in week 00. The modified command %NW where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.

%x The locale's date representation. The modified command %Ex produces the locale's alternate date representation.

%X The locale's time representation. The modified command %Ex produces the locale's alternate time representation.

%y The last two decimal digits of the year. If the century is not otherwise specified (e.g. with %C), values in the range [69 - 99] are presumed to refer to the years [1969 - 1999], and values in the range [00 - 68] are presumed to refer to the years [2000 - 2068]. The modified command %Ny where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands %Ey and %Oy interpret the locale's alternative representation.

%Y The year as a decimal number. The modified command %NY where N is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 4. Leading zeroes are permitted but not required. The modified command %EY interprets the locale's alternative representation.

%z The offset from UTC in the ISO 8601 format. For example -0430 refers to 4 hours 30 minutes behind UTC. The modified commands %Ez and %Ez parse a : between the hours and minutes and leading zeroes on the hour field are optional: -4:30.

%Z The time zone abbreviation or name. A single word is parsed. This word can only contain characters from the basic source character set ([lex.charset] in the C++ standard) that are alphanumeric, or one of '_', '/', '-' or '+'.

%% A % character is extracted.

Back to TOC

Add new section [time.calendar] after 23.17.9 Clocks [time.parse]:

23.17.10 The civil calendar [time.calendar]

The types in this subclause describe the civil (Gregorian) calendar and its relationship to sys_days and local_days.

23.17.10.1 Class last_spec [time.calendar.last]

The struct last_spec is used in conjunction with other calendar types to specify the last in a sequence. For example, depending on context, it can represent the last day of a month, or the last day of the week of a month.

There is an constexpr object of this type named last in the chrono_literals namespace.
struct last_spec
{
    explicit last_spec() = default;
};
23.17.10.2 Class day [time.calendar.day]

day represents a day of a month. It normally holds values in the range 1 to 31. However it may hold non-negative values outside this range. It can be constructed with any unsigned value, which will be subsequently truncated to fit into day's unspecified internal storage. day is equality and less-than comparable, and participates in basic arithmetic with days representing the quantity between any two day's. One can form a day literal with d. And one can stream out a day. day has explicit conversions to and from unsigned.
class day
{
    unsigned char d_;  // exposition only
public:
    day() = default;
    explicit constexpr day(unsigned d) noexcept;

    constexpr day& operator++()    noexcept;
    constexpr day  operator++(int) noexcept;
    constexpr day& operator--()    noexcept;
    constexpr day  operator--(int) noexcept;

    constexpr day& operator+=(const days& d) noexcept;
    constexpr day& operator-=(const days& d) noexcept;

    constexpr explicit operator unsigned() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const day& x, const day& y) noexcept;
constexpr bool operator!=(const day& x, const day& y) noexcept;
constexpr bool operator< (const day& x, const day& y) noexcept;
constexpr bool operator> (const day& x, const day& y) noexcept;
constexpr bool operator<=(const day& x, const day& y) noexcept;
constexpr bool operator>=(const day& x, const day& y) noexcept;

constexpr day  operator+(const day&  x, const days& y) noexcept;
constexpr day  operator+(const days& x, const day&  y) noexcept;
constexpr day  operator-(const day&  x, const days& y) noexcept;
constexpr days operator-(const day&  x, const day&  y) noexcept;

template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const day& d);

template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const day& d);

template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            day& d, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
day is a trivially copyable class type.
day is a standard-layout class type.
explicit constexpr day::day(unsigned d) noexcept;
Effects: Constructs an object of type day by constructing d_ with d.
constexpr day& day::operator++() noexcept;
Effects: ++d_.

Returns: *this.
constexpr day day::operator++(int) noexcept;
Effects: ++(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr day& day::operator--() noexcept;
Effects: --d_.

Returns: *this.
constexpr day day::operator--(int) noexcept;
Effects: --(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr day& day::operator+=(const days& d) noexcept;
Effects: *this = *this + d.

Returns: *this.
constexpr day& day::operator-=(const days& d) noexcept;
Effects: *this = *this - d.

Returns: *this.
constexpr explicit day::operator unsigned() const noexcept;
Returns: d_.
constexpr bool day::ok() const noexcept;
Returns: 1 <= d_ && d_ <= 31.
constexpr bool operator==(const day& x, const day& y) noexcept;
Returns: unsigned{x} == unsigned{y}.
constexpr bool operator!=(const day& x, const day& y) noexcept;
Returns: !(x == y).
constexpr bool operator< (const day& x, const day& y) noexcept;
Returns: unsigned{x} < unsigned{y}.
constexpr bool operator> (const day& x, const day& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const day& x, const day& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const day& x, const day& y) noexcept;
Returns: !(x < y).
constexpr day  operator+(const day&  x, const days& y) noexcept;
Returns: day{unsigned{x} + y.count()}.
constexpr day  operator+(const days& x, const day&  y) noexcept;
Returns: y + x.
constexpr day  operator-(const day&  x, const days& y) noexcept;
Returns: x + -y.
constexpr days operator-(const day&  x, const day&  y) noexcept;
Returns: days{static_cast<days::rep>(unsigned{x} - unsigned{y})}.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const day& d);
Effects: Inserts a decimal integral text representation of d into os. Single digit values are prefixed with '0'.

Returns: os.
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const day& d);
Effects: Streams d into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            day& d, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the day d using the format flags as specified in [time.parse].

If the parse fails to decode a valid day, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.
constexpr day operator "" d(unsigned long long d) noexcept;
Returns: day{static_cast<unsigned>(d)}.

23.17.10.3 Class month [time.calendar.month]

month represents a month of a year. It normally holds values in the range 1 to 12. However it may hold non-negative values outside this range. It can be constructed with any unsigned value, which will be subsequently truncated to fit into month's unspecified internal storage. month is equality and less-than comparable, and participates in basic arithmetic with months representing the quantity between any two month's. One can stream out a month. month has explicit conversions to and from unsigned. There are 12 month constants, one for each month of the year in the chrono_literals namespace.
class month
{
    unsigned char m_;  // exposition only
public:
    month() = default;
    explicit constexpr month(unsigned m) noexcept;

    constexpr month& operator++()    noexcept;
    constexpr month  operator++(int) noexcept;
    constexpr month& operator--()    noexcept;
    constexpr month  operator--(int) noexcept;

    constexpr month& operator+=(const months& m) noexcept;
    constexpr month& operator-=(const months& m) noexcept;

    constexpr explicit operator unsigned() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const month& x, const month& y) noexcept;
constexpr bool operator!=(const month& x, const month& y) noexcept;
constexpr bool operator< (const month& x, const month& y) noexcept;
constexpr bool operator> (const month& x, const month& y) noexcept;
constexpr bool operator<=(const month& x, const month& y) noexcept;
constexpr bool operator>=(const month& x, const month& y) noexcept;

constexpr month  operator+(const month&  x, const months& y) noexcept;
constexpr month  operator+(const months& x,  const month& y) noexcept;
constexpr month  operator-(const month&  x, const months& y) noexcept;
constexpr months operator-(const month&  x,  const month& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month& m);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month& m);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                month& m, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
month is a trivially copyable class type.
month is a standard-layout class type.
explicit constexpr month::month(unsigned m) noexcept;
Effects: Constructs an object of type month by constructing m_ with m.
constexpr month& month::operator++() noexcept;
Effects: If m_ < 12, ++m_. Otherwise sets m_ to 1.

Returns: *this.
constexpr month month::operator++(int) noexcept;
Effects: ++(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr month& month::operator--() noexcept;
Effects: If m_ > 1, --m_. Otherwise sets m_ to 12.

Returns: *this.
constexpr month month::operator--(int) noexcept;
Effects: --(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr month& month::operator+=(const months& m) noexcept;
Effects: *this = *this + m.

Returns: *this.
constexpr month& month::operator-=(const months& m) noexcept;
Effects: *this = *this - m.

Returns: *this.
constexpr explicit month::operator unsigned() const noexcept;
Returns: m_.
constexpr bool month::ok() const noexcept;
Returns: 1 <= m_ && m_ <= 12.
constexpr bool operator==(const month& x, const month& y) noexcept;
Returns: unsigned{x} == unsigned{y}.
constexpr bool operator!=(const month& x, const month& y) noexcept;
Returns: !(x == y).
constexpr bool operator< (const month& x, const month& y) noexcept;
Returns: unsigned{x} < unsigned{y}.
constexpr bool operator> (const month& x, const month& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const month& x, const month& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const month& x, const month& y) noexcept;
Returns: !(x < y).
constexpr month  operator+(const month&  x, const months& y) noexcept;
Returns: A month for which ok() == true and is found as if by incrementing (or decrementing if y < months{0}) x, y times. If month.ok() == false prior to this operation, behaves as if *this is first brought into the range [1, 12] by modular arithmetic. [Note: For example month{0} becomes month{12}, and month{13} becomes month{1}. — end note]

Complexity: O(1) with respect to the value of y. [Note: Repeated increments or decrements is not a valid implementation. — end note]

Example: feb + months{11} == jan.
constexpr month  operator+(const months& x, const month&  y) noexcept;
Returns: y + x.
constexpr month  operator-(const month&  x, const months& y) noexcept;
Returns: x + -y.
constexpr months operator-(const month&  x, const month&  y) noexcept;
Requires: x.ok() == true and y.ok() == true.

Returns: A value of months in the range of months{0} to months{11} inclusive.

Remarks: The returned value m shall satisfy the equality: y + m == x.

Example: jan - feb == months{11} .
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const month& m);
Effects: If ok() == true outputs the same string that would be output for the month field by asctime. Otherwise outputs unsigned{m} << " is not a valid month".

Returns: os.
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month& m);
Effects: Streams m into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            month& m, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the month m using the format flags as specified in [time.parse].

If the parse fails to decode a valid month, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

23.17.10.4 Class year [time.calendar.year]

year represents a year in the civil calendar. It shall represent values in the range [min(), max()]. It can be constructed with any int value, which will be subsequently truncated to fit into year's internal unspecified storage. year is equality and less-than comparable, and participates in basic arithmetic with years representing the quantity between any two year's. One can form a year literal with y. And one can stream out a year. year has explicit conversions to and from int.
class year
{
    short y_;  // exposition only
public:
    year() = default;
    explicit constexpr year(int y) noexcept;

    constexpr year& operator++()    noexcept;
    constexpr year  operator++(int) noexcept;
    constexpr year& operator--()    noexcept;
    constexpr year  operator--(int) noexcept;

    constexpr year& operator+=(const years& y) noexcept;
    constexpr year& operator-=(const years& y) noexcept;

    constexpr year operator+() const noexcept;
    constexpr year operator-() const noexcept;

    constexpr bool is_leap() const noexcept;

    constexpr explicit operator int() const noexcept;
    constexpr bool ok() const noexcept;

    static constexpr year min() noexcept;
    static constexpr year max() noexcept;
};

constexpr bool operator==(const year& x, const year& y) noexcept;
constexpr bool operator!=(const year& x, const year& y) noexcept;
constexpr bool operator< (const year& x, const year& y) noexcept;
constexpr bool operator> (const year& x, const year& y) noexcept;
constexpr bool operator<=(const year& x, const year& y) noexcept;
constexpr bool operator>=(const year& x, const year& y) noexcept;

constexpr year  operator+(const year&  x, const years& y) noexcept;
constexpr year  operator+(const years& x, const year&  y) noexcept;
constexpr year  operator-(const year&  x, const years& y) noexcept;
constexpr years operator-(const year&  x, const year&  y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year& y);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year& y);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year& y, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
year is a trivially copyable class type.
year is a standard-layout class type.
explicit constexpr year::year(int y) noexcept;
Effects: Constructs an object of type year by constructing y_ with y.
constexpr year& year::operator++() noexcept;
Effects: ++y_.

Returns: *this.
constexpr year year::operator++(int) noexcept;
Effects: ++(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr year& year::operator--() noexcept;
Effects: --y_.

Returns: *this.
constexpr year year::operator--(int) noexcept;
Effects: --(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr year& year::operator+=(const years& y) noexcept;
Effects: *this = *this + y.

Returns: *this.
constexpr year& year::operator-=(const years& y) noexcept;
Effects: *this = *this - y.

Returns: *this.
constexpr year& year::operator+() const noexcept;
Returns: *this.
constexpr year& year::operator-() const noexcept;
Returns: year{-y_}.
constexpr bool year::is_leap() const noexcept;
Effects: Equivalent to: return y_ % 4 == 0 && (y_ % 100 != 0 || y_ % 400 == 0);
constexpr explicit year::operator int() const noexcept;
Returns: y_.
constexpr bool year::ok() const noexcept;
Returns: min() <= y_ && y_ <= max().
static constexpr year year::min() noexcept;
Returns: -32767.
static constexpr year year::max() noexcept;
Returns: 32767.
constexpr bool operator==(const year& x, const year& y) noexcept;
Returns: int{x} == int{y}.
constexpr bool operator!=(const year& x, const year& y) noexcept;
Returns: !(x == y).
constexpr bool operator< (const year& x, const year& y) noexcept;
Returns: int{x} < int{y}.
constexpr bool operator> (const year& x, const year& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const year& x, const year& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const year& x, const year& y) noexcept;
Returns: !(x < y).
constexpr year  operator+(const year&  x, const years& y) noexcept;
Returns: year{int{x} + y.count()}.
constexpr year  operator+(const years& x, const year&  y) noexcept;
Returns: y + x.
constexpr year  operator-(const year&  x, const years& y) noexcept;
Returns: x + -y.
constexpr years operator-(const year&  x, const year&  y) noexcept;
Returns: years{int{x} - int{y}}.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year& y);
Effects: Inserts a signed decimal integral text representation of y into os. If the year is in the range [-999, 999], prefixes the year with '0' to four digits. If the year is negative, prefixes with '-'.

Returns: os.
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year& y);
Effects: Streams y into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            year& y, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the year y using the format flags as specified in [time.parse].

If the parse fails to decode a valid year, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.
constexpr year operator "" y(unsigned long long y) noexcept;
Returns: year{static_cast<int>(y)}.

23.17.10.5 Class weekday [time.calendar.weekday]

weekday represents a day of the week in the civil calendar. It normally holds values in the range 0 to 6, corresponding to Sunday through Saturday. However it may hold non-negative values outside this range. It can be constructed with any unsigned value, which will be subsequently truncated to fit into weekday's unspecified internal storage. weekday is equality comparable. weekday is not less-than comparable because there is no universal consensus on which day is the first day of the week. This design chooses the encoding of 0 to 6 to represent Sunday through Saturday only because this is consistent with existing C and C++ practice. However weekday's comparison and arithmetic operations treat the days of the week as a circular range, with no beginning and no end. One can stream out a weekday. weekday has explicit conversions to and from unsigned. There are 7 weekday constants, one for each day of the week in the chrono_literals namespace.

A weekday can be implicitly constructed from a sys_days. This is the computation that discovers the day of the week of an arbitrary date.

A weekday can be indexed with either unsigned or last. This produces new types which represent the first, second, third, fourth, fifth or last weekdays of a month.
class weekday
{
    unsigned char wd_;  // exposition only
public:
    weekday() = default;
    explicit constexpr weekday(unsigned wd) noexcept;
    constexpr weekday(const sys_days& dp) noexcept;
    constexpr explicit weekday(const local_days& dp) noexcept;

    constexpr weekday& operator++()    noexcept;
    constexpr weekday  operator++(int) noexcept;
    constexpr weekday& operator--()    noexcept;
    constexpr weekday  operator--(int) noexcept;

    constexpr weekday& operator+=(const days& d) noexcept;
    constexpr weekday& operator-=(const days& d) noexcept;

    constexpr explicit operator unsigned() const noexcept;
    constexpr bool ok() const noexcept;

    constexpr weekday_indexed operator[](unsigned index) const noexcept;
    constexpr weekday_last    operator[](last_spec) const noexcept;
};

constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;

constexpr weekday operator+(const weekday& x, const days&    y) noexcept;
constexpr weekday operator+(const days&    x, const weekday& y) noexcept;
constexpr weekday operator-(const weekday& x, const days&    y) noexcept;
constexpr days    operator-(const weekday& x, const weekday& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday& wd);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const weekday& wd);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
weekday is a trivially copyable class type.
weekday is a standard-layout class type.
explicit constexpr weekday::weekday(unsigned wd) noexcept;
Effects: Constructs an object of type weekday by constructing wd_ with wd.
constexpr weekday(const sys_days& dp) noexcept;
Effects: Constructs an object of type weekday by computing what day of the week corresponds to the sys_days dp, and representing that day of the week in wd_.

Example: If dp represents 1970-01-01, the constructed weekday represents Thursday by storing 4 in wd_.
constexpr explicit weekday(const local_days& dp) noexcept;
Effects: Constructs an object of type weekday by computing what day of the week corresponds to the local_days dp, and representing that day of the week in wd_.

The value after construction shall be identical to that constructed from sys_days{dp.time_since_epoch()}.
constexpr weekday& weekday::operator++() noexcept;
Effects: If wd_ != 6, ++wd_. Otherwise sets wd_ to 0.

Returns: *this.
constexpr weekday weekday::operator++(int) noexcept;
Effects: ++(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr weekday& weekday::operator--() noexcept;
Effects: If wd_ != 0, --wd_. Otherwise sets wd_ to 6.

Returns: *this.
constexpr weekday weekday::operator--(int) noexcept;
Effects: --(*this).

Returns: A copy of *this as it existed on entry to this member function.
constexpr weekday& weekday::operator+=(const days& d) noexcept;
Effects: *this = *this + d.

Returns: *this.
constexpr weekday& weekday::operator-=(const days& d) noexcept;
Effects: *this = *this - d.

Returns: *this.
constexpr explicit weekday::operator unsigned() const noexcept;
Returns: wd_.
constexpr bool weekday::ok() const noexcept;
Returns: wd_ <= 6.
constexpr weekday_indexed weekday::operator[](unsigned index) const noexcept;
Returns: {*this, index}.
constexpr weekday_last weekday::operator[](last_spec) const noexcept;
Returns: weekday_last{*this}.
constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
Returns: unsigned{x} == unsigned{y}.
constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;
Returns: !(x == y).
constexpr weekday  operator+(const weekday&  x, const days& y) noexcept;
Returns: A weekday for which ok() == true and is found as if by incrementing (or decrementing if y < days{0}) x, y times. If weekday.ok() == false prior to this operation, behaves as if *this is first brought into the range [0, 6] by modular arithmetic. [Note: For example weekday{7} becomes weekday{0}. — end note]

Complexity: O(1) with respect to the value of y. [Note: Repeated increments or decrements is not a valid implementation. — end note]

Example: mon + days{6} == sun.
constexpr weekday  operator+(const days& x, const weekday&  y) noexcept;
Returns: y + x.
constexpr weekday  operator-(const weekday&  x, const days& y) noexcept;
Returns: x + -y.
constexpr days operator-(const weekday&  x, const weekday&  y) noexcept;
Requires: x.ok() == true and y.ok() == true.

Returns: A value of days in the range of days{0} to days{6} inclusive.

Remarks: The returned value d shall satisfy the equality: y + d == x.

Example: sun - mon == days{6}.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
Effects: If ok() == true outputs the same string that would be output for the weekday field by asctime. Otherwise outputs unsigned{wd} << " is not a valid weekday".

Returns: os.
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const weekday& wd);
Effects: Streams wd into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the weekday wd using the format flags as specified in [time.parse].

If the parse fails to decode a valid weekday, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

23.17.10.6 Class weekday_indexed [time.calendar.weekday_indexed]

weekday_indexed represents a weekday and a small index in the range 1 to 5. This class is used to represent the first, second, third, fourth or fifth weekday of a month. It is most easily constructed by indexing a weekday.

[Example:
constexpr auto wdi = sun[2];  // wdi is the second Sunday of an as yet unspecified month
static_assert(wdi.weekday() == sun);
static_assert(wdi.index() == 2);
— end example:]
class weekday_indexed
{
    chrono::weekday  wd_;     // exposition only
    unsigned char    index_;  // exposition only

public:
    weekday_indexed() = default;
    constexpr weekday_indexed(const chrono::weekday& wd, unsigned index) noexcept;

    constexpr chrono::weekday weekday() const noexcept;
    constexpr unsigned        index()   const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const weekday_indexed& x, const weekday_indexed& y) noexcept;
constexpr bool operator!=(const weekday_indexed& x, const weekday_indexed& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday_indexed& wdi);
weekday_indexed is a trivially copyable class type.
weekday_indexed is a standard-layout class type.
constexpr weekday_indexed::weekday_indexed(const chrono::weekday& wd, unsigned index) noexcept;
Effects: Constructs an object of type weekday_indexed by constructing wd_ with wd and index_ with index.
constexpr weekday weekday_indexed::weekday() const noexcept;
Returns: wd_.
constexpr unsigned weekday_indexed::index() const noexcept;
Returns: index_.
constexpr bool weekday_indexed::ok() const noexcept;
Returns: wd_.ok() && 1 <= index_ && index_ <= 5.
constexpr bool operator==(const weekday_indexed& x, const weekday_indexed& y) noexcept;
Returns: x.weekday() == y.weekday() && x.index() == y.index().
constexpr bool operator!=(const weekday_indexed& x, const weekday_indexed& y) noexcept;
Returns: !(x == y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const weekday_indexed& wdi);
Effects: Equivalent to:
return os << wdi.weekday() << '[' << wdi.index() << ']';
23.17.10.7 Class weekday_last [time.calendar.weekday_last]

weekday_last represents the last weekday of a month. It is most easily constructed by indexing a weekday with last.

[Example:
constexpr auto wdl = sun[last];  // wdl is the last Sunday of an as yet unspecified month
static_assert(wdl.weekday() == sun);
— end example:]
class weekday_last
{
    chrono::weekday wd_;  // exposition only

public:
    explicit constexpr weekday_last(const chrono::weekday& wd) noexcept;

    constexpr chrono::weekday weekday() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const weekday_last& x, const weekday_last& y) noexcept;
constexpr bool operator!=(const weekday_last& x, const weekday_last& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const weekday_last& wdl);
weekday_last is a trivially copyable class type.
weekday_last is a standard-layout class type.
explicit constexpr weekday_last::weekday_last(const chrono::weekday& wd) noexcept;
Effects: Constructs an object of type weekday_last by constructing wd_ with wd.
constexpr weekday weekday_last::weekday() const noexcept;
Returns: wd_.
constexpr bool weekday_last::ok() const noexcept;
Returns: wd_.ok().
constexpr bool operator==(const weekday_last& x, const weekday_last& y) noexcept;
Returns: x.weekday() == y.weekday().
constexpr bool operator!=(const weekday_last& x, const weekday_last& y) noexcept;
Returns: !(x == y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const weekday_last& wdl);
Effects: Equivalent to:
return os << wdi.weekday() << "[last]";
23.17.10.8 Class month_day [time.calendar.month_day]

month_day represents a specific day of a specific month, but with an unspecified year. One can observe the different components. One can assign a new value. month_day is equality comparable and less-than comparable. One can stream out a month_day.
class month_day
{
    chrono::month m_;  // exposition only
    chrono::day   d_;  // exposition only

public:
    month_day() = default;
    constexpr month_day(const chrono::month& m, const chrono::day& d) noexcept;

    constexpr chrono::month month() const noexcept;
    constexpr chrono::day   day()   const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const month_day& x, const month_day& y) noexcept;
constexpr bool operator!=(const month_day& x, const month_day& y) noexcept;
constexpr bool operator< (const month_day& x, const month_day& y) noexcept;
constexpr bool operator> (const month_day& x, const month_day& y) noexcept;
constexpr bool operator<=(const month_day& x, const month_day& y) noexcept;
constexpr bool operator>=(const month_day& x, const month_day& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_day& md);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month_day& md);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                month_day& md, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
month_day is a trivially copyable class type.
month_day is a standard-layout class type.
constexpr month_day::month_day(const chrono::month& m, const chrono::day& d) noexcept;
Effects: Constructs an object of type month_day by constructing m_ with m, and d_ with d.
constexpr month month_day::month() const noexcept;
Returns: m_.
constexpr day month_day::day() const noexcept;
Returns: d_.
constexpr bool month_day::ok() const noexcept;
Returns: true if m_.ok() is true, and if 1d <= d_, and if d_ <= the number of days in month m_. For m_ == feb the number of days is considered to be 29. Otherwise returns false.
constexpr bool operator==(const month_day& x, const month_day& y) noexcept;
Returns: x.month() == y.month() && x.day() == y.day()
constexpr bool operator!=(const month_day& x, const month_day& y) noexcept;
Returns: !(x == y)
constexpr bool operator< (const month_day& x, const month_day& y) noexcept;
Returns: If x.month() < y.month() returns true. Else if x.month() > y.month() returns false. Else returns x.day() < y.day().
constexpr bool operator> (const month_day& x, const month_day& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const month_day& x, const month_day& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const month_day& x, const month_day& y) noexcept;
Returns: !(x < y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const month_day& md);
Effects: Equivalent to:
return os << md.month() << '/' << md.day();
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const month_day& md);
Effects: Streams md into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            month_day& md, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the month_day md using the format flags as specified in [time.parse].

If the parse fails to decode a valid month_day, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

23.17.10.9 Class month_day_last [time.calendar.month_day_last]

month_day_last represents the last day of a month. It is most easily constructed using the expression m/last or last/m, where m is an expression with type month.

[Example:
constexpr auto mdl = feb/last;  // mdl is the last day of February of an as yet unspecified year
static_assert(mdl.month() == feb);
— end example:]
class month_day_last
{
    chrono::month m_;  // exposition only

public:
    constexpr explicit month_day_last(const chrono::month& m) noexcept;

    constexpr chrono::month month() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator!=(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator< (const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator> (const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator<=(const month_day_last& x, const month_day_last& y) noexcept;
constexpr bool operator>=(const month_day_last& x, const month_day_last& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_day_last& mdl);
month_day_last is a trivially copyable class type.
month_day_last is a standard-layout class type.
constexpr explicit month_day_last::month_day_last(const chrono::month& m) noexcept;
Effects: Constructs an object of type month_day_last by constructing m_ with m.
constexpr month month_day_last::month() const noexcept;
Returns: m_.
constexpr bool month_day_last::ok() const noexcept;
Returns: m_.ok().
constexpr bool operator==(const month_day_last& x, const month_day_last& y) noexcept;
Returns: x.month() == y.month().
constexpr bool operator!=(const month_day_last& x, const month_day_last& y) noexcept;
Returns: !(x == y)
constexpr bool operator< (const month_day_last& x, const month_day_last& y) noexcept;
Returns: x.month() < y.month().
constexpr bool operator> (const month_day_last& x, const month_day_last& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const month_day_last& x, const month_day_last& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const month_day_last& x, const month_day_last& y) noexcept;
Returns: !(x < y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const month_day_last& mdl);
Effects: Equivalent to:
return os << mdl.month() << "/last";
23.17.10.10 Class month_weekday [time.calendar.month_weekday]

month_weekday represents the nth weekday of a month, of an as yet unspecified year. To do this the month_weekday stores a month and a weekday_indexed.
class month_weekday
{
    chrono::month           m_;    // exposition only
    chrono::weekday_indexed wdi_;  // exposition only
public:
    constexpr month_weekday(const chrono::month& m, const chrono::weekday_indexed& wdi) noexcept;

    constexpr chrono::month           month()           const noexcept;
    constexpr chrono::weekday_indexed weekday_indexed() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const month_weekday& x, const month_weekday& y) noexcept;
constexpr bool operator!=(const month_weekday& x, const month_weekday& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_weekday& mwd);
month_weekday is a trivially copyable class type.
month_weekday is a standard-layout class type.
constexpr month_weekday::month_weekday(const chrono::month& m, const chrono::weekday_indexed& wdi) noexcept;
Effects: Constructs an object of type month_weekday by constructing m_ with m, and wdi_ with wdi.
constexpr month month_weekday::month() const noexcept;
Returns: m_.
constexpr weekday_indexed month_weekday::weekday_indexed() const noexcept;
Returns: wdi_.
constexpr bool month_weekday::ok() const noexcept;
Returns: m_.ok() && wdi_.ok().
constexpr bool operator==(const month_weekday& x, const month_weekday& y) noexcept;
Returns: x.month() == y.month() && x.weekday_indexed() == y.weekday_indexed().
constexpr bool operator!=(const month_weekday& x, const month_weekday& y) noexcept;
Returns: !(x == y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const month_weekday& mwd);
Effects: Equivalent to:
return os << mwd.month() << '/' << mwd.weekday_indexed();
23.17.10.11 Class month_weekday_last [time.calendar.month_weekday_last]

month_weekday_last represents the last weekday of a month, of an as yet unspecified year. To do this the month_weekday_last stores a month and a weekday_last.
class month_weekday_last
{
    chrono::month        m_;     // exposition only
    chrono::weekday_last wdl_;   // exposition only
public:
    constexpr month_weekday_last(const chrono::month& m,
                                 const chrono::weekday_last& wdl) noexcept;

    constexpr chrono::month        month()        const noexcept;
    constexpr chrono::weekday_last weekday_last() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const month_weekday_last& x, const month_weekday_last& y) noexcept;
constexpr bool operator!=(const month_weekday_last& x, const month_weekday_last& y) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const month_weekday_last& mwdl);
month_weekday_last is a trivially copyable class type.
month_weekday_last is a standard-layout class type.
constexpr month_weekday_last::month_weekday_last(const chrono::month& m,
                                                 const chrono::weekday_last& wdl) noexcept;
Effects: Constructs an object of type month_weekday_last by constructing m_ with m, and wdl_ with wdl.
constexpr month month_weekday_last::month() const noexcept;
Returns: m_.
constexpr weekday_last month_weekday_last::weekday_last() const noexcept;
Returns: wdl_.
constexpr bool month_weekday_last::ok() const noexcept;
Returns: m_.ok() && wdl_.ok().
constexpr bool operator==(const month_weekday_last& x, const month_weekday_last& y) noexcept;
Returns: x.month() == y.month() && x.weekday_last() == y.weekday_last().
constexpr bool operator!=(const month_weekday_last& x, const month_weekday_last& y) noexcept;
Returns: !(x == y).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const month_weekday_last& mwdl);
Effects: Equivalent to:
return os << mwdl.month() << '/' << mwdl.weekday_last();
23.17.10.12 Class year_month [time.calendar.year_month]

year_month represents a specific month of a specific year, but with an unspecified day. year_month is a field-based time point with a resolution of months. One can observe the different components. One can assign a new value. year_month is equality comparable and less-than comparable. One can stream out a year_month.
class year_month
{
    chrono::year  y_;  // exposition only
    chrono::month m_;  // exposition only

public:
    year_month() = default;
    constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;

    constexpr chrono::year  year()  const noexcept;
    constexpr chrono::month month() const noexcept;

    constexpr year_month& operator+=(const months& dm) noexcept;
    constexpr year_month& operator-=(const months& dm) noexcept;
    constexpr year_month& operator+=(const years& dy)  noexcept;
    constexpr year_month& operator-=(const years& dy)  noexcept;

    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
constexpr bool operator!=(const year_month& x, const year_month& y) noexcept;
constexpr bool operator< (const year_month& x, const year_month& y) noexcept;
constexpr bool operator> (const year_month& x, const year_month& y) noexcept;
constexpr bool operator<=(const year_month& x, const year_month& y) noexcept;
constexpr bool operator>=(const year_month& x, const year_month& y) noexcept;

constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
constexpr months operator-(const year_month& x, const year_month& y) noexcept;
constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month& ym);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month& ym);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
year_month is a trivially copyable class type.
year_month is a standard-layout class type.
constexpr year_month::year_month(const chrono::year& y, const chrono::month& m) noexcept;
Effects: Constructs an object of type year_month by constructing y_ with y, and m_ with m.
constexpr year year_month::year() const noexcept;
Returns: y_.
constexpr month year_month::month() const noexcept;
Returns: m_.
constexpr year_month& operator+=(const months& dm) noexcept;
Effects: *this = *this + dm.

Returns: *this.
constexpr year_month& operator-=(const months& dm) noexcept;
Effects: *this = *this - dm.

Returns: *this.
constexpr year_month& operator+=(const years& dy) noexcept;
Effects: *this = *this + dy.

Returns: *this.
constexpr year_month& operator-=(const years& dy) noexcept;
Effects: *this = *this - dy.

Returns: *this.
constexpr bool year_month::ok() const noexcept;
Returns: y_.ok() && m_.ok().
constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
Returns: x.year() == y.year() && x.month() == y.month()
constexpr bool operator!=(const year_month& x, const year_month& y) noexcept;
Returns: !(x == y)
constexpr bool operator< (const year_month& x, const year_month& y) noexcept;
Returns: If x.year() < y.year() returns true. Else if x.year() > y.year() returns false. Else returns x.month() < y.month().
constexpr bool operator> (const year_month& x, const year_month& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const year_month& x, const year_month& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const year_month& x, const year_month& y) noexcept;
Returns: !(x < y).
constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
Returns: A year_month value z such that z - ym == dm.

Complexity: O(1) with respect to the value of dm.
constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
Returns: ym + dm.
constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
Returns: ym + -dm.
constexpr months operator-(const year_month& x, const year_month& y) noexcept;
Returns: x.year() - y.year() + months{static_cast<int>(unsigned{x.month()}) - static_cast<int>(unsigned{y.month()})}.
constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
Returns: (ym.year() + dy) / ym.month().
constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
Returns: ym + dy.
constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;
Returns: ym + -dy.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month& ym);
Effects: Equivalent to:
return os << ym.year() << '/' << ym.month();
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month& ym);
Effects: Streams ym into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the year_month ym using the format flags as specified in [time.parse].

If the parse fails to decode a valid year_month, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

23.17.10.13 Class year_month_day [time.calendar.year_month_day]

year_month_day represents a specific year, month, and day. year_month_day is a field-based time point with a resolution of days. One can observe each field. year_month_day supports years and months oriented arithmetic, but not days oriented arithmetic. For the latter, there is a conversion to sys_days which efficiently supports days oriented arithmetic. There is also a conversion from sys_days. year_month_day is equality and less-than comparable.
class year_month_day
{
    chrono::year  y_;  // exposition only
    chrono::month m_;  // exposition only
    chrono::day   d_;  // exposition only

public:
    year_month_day() = default;
    constexpr year_month_day(const chrono::year& y, const chrono::month& m, const chrono::day& d) noexcept;
    constexpr year_month_day(const year_month_day_last& ymdl) noexcept;
    constexpr year_month_day(const sys_days& dp) noexcept;
    constexpr explicit year_month_day(const local_days& dp) noexcept;

    constexpr year_month_day& operator+=(const months& m) noexcept;
    constexpr year_month_day& operator-=(const months& m) noexcept;
    constexpr year_month_day& operator+=(const years& y)  noexcept;
    constexpr year_month_day& operator-=(const years& y)  noexcept;

    constexpr chrono::year  year()  const noexcept;
    constexpr chrono::month month() const noexcept;
    constexpr chrono::day   day()   const noexcept;

    constexpr          operator sys_days()   const noexcept;
    constexpr explicit operator local_days() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator!=(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator< (const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator> (const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator<=(const year_month_day& x, const year_month_day& y) noexcept;
constexpr bool operator>=(const year_month_day& x, const year_month_day& y) noexcept;

constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;
constexpr year_month_day operator+(const years& dy, const year_month_day& ymd) noexcept;
constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;
constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_day& ymd);

template <class charT, class traits>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month_day& ymd);

template <class charT, class traits, class Alloc = allocator<charT>>
    basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                year_month_day& ymd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);
year_month_day is a trivially copyable class type.
year_month_day is a standard-layout class type.
constexpr year_month_day::year_month_day(const chrono::year& y, const chrono::month& m, const chrono::day& d) noexcept;
Effects: Constructs an object of type year_month_day by constructing y_ with y, m_ with m, and, d_ with d.
constexpr year_month_day::year_month_day(const year_month_day_last& ymdl) noexcept;
Effects: Constructs an object of type year_month_day by constructing y_ with ymdl.year(), m_ with ymdl.month(), and, d_ with ymdl.day().

Note: This conversion from year_month_day_last to year_month_day is more efficient than converting a year_month_day_last to a sys_days, and then converting that sys_days to a year_month_day.
constexpr year_month_day::year_month_day(const sys_days& dp) noexcept;
Effects: Constructs an object of type year_month_day which corresponds to the date represented by dp.

Remarks: For any value of year_month_day, ymd, for which ymd.ok() is true, this equality will also be true: ymd == year_month_day{sys_days{ymd}}.
constexpr explicit year_month_day::year_month_day(const local_days& dp) noexcept;
Effects: Constructs an object of type year_month_day which corresponds to the date represented by dp.

Remarks: Equivalent to constructing with sys_days{dp.time_since_epoch()}.
constexpr year_month_day& year_month_day::operator+=(const months& m) noexcept;
Effects: *this = *this + m;.

Returns: *this.
constexpr year_month_day& year_month_day::operator-=(const months& m) noexcept;
Effects: *this = *this - m;.

Returns: *this.
constexpr year_month_day& year_month_day::operator+=(const years& y) noexcept;
Effects: *this = *this + y;.

Returns: *this.
constexpr year_month_day& year_month_day::operator-=(const years& y) noexcept;
Effects: *this = *this - y;.

Returns: *this.
constexpr year year_month_day::year() const noexcept;
Returns: y_.
constexpr month year_month_day::month() const noexcept;
Returns: m_.
constexpr day year_month_day::day() const noexcept;
Returns: d_.
constexpr year_month_day::operator sys_days() const noexcept;
Requires: y_.ok() && m_.ok() == true.

Returns: If ok(), returns a sys_days holding a count of days from the sys_days epoch to *this (a negative value if *this represents a date prior to the sys_days epoch). Otherwise returns a sys_days which is offset from sys_days{y_/m_/last} by the number of days d_ is offset from sys_days{y_/m_/last}.day().

Remarks: A sys_days in the range [days{-12687428}, days{11248737}] which is converted to a year_month_day, shall have the same value when converted back to a sys_days.

[Example:
static_assert(year_month_day{sys_days{2017y/jan/0}}  == 2016y/dec/31);
static_assert(year_month_day{sys_days{2017y/jan/31}} == 2017y/jan/31);
static_assert(year_month_day{sys_days{2017y/jan/32}} == 2017y/feb/1);
—end example]
constexpr explicit year_month_day::operator local_days() const noexcept;
Requires: y_.ok() && m_.ok() == true.

Effects: Equivalent to:
return local_days{sys_days{*this}.time_since_epoch()};
constexpr bool year_month_day::ok() const noexcept;
Returns: If y_.ok() is true, and m_.ok() is true, and d_ is in the range [1d, (y_/m_/last).day()], then returns true, else returns false.
constexpr bool operator==(const year_month_day& x, const year_month_day& y) noexcept;
Returns: x.year() == y.year() && x.month() == y.month() && x.day() == y.day().
constexpr bool operator!=(const year_month_day& x, const year_month_day& y) noexcept;
Returns: !(x == y).
constexpr bool operator< (const year_month_day& x, const year_month_day& y) noexcept;
Returns: If x.year() < y.year(), returns true. Else if x.year() > y.year() returns false. Else if x.month() < y.month(), returns true. Else if x.month() > y.month(), returns false. Else returns x.day() < y.day().
constexpr bool operator> (const year_month_day& x, const year_month_day& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const year_month_day& x, const year_month_day& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const year_month_day& x, const year_month_day& y) noexcept;
Returns: !(x < y).
constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
Requires: ymd.month().ok() is true.

Returns: (ymd.year() / ymd.month() + dm) / ymd.day().

Remarks: If ymd.day() is in the range [1d, 28d], the resultant year_month_day shall return true from ok().
constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
Returns: ymd + dm.
constexpr year_month_day operator-(const year_month_day& ymd, const months& dm) noexcept;
Returns: ymd + (-dm).
constexpr year_month_day operator+(const year_month_day& ymd, const years& dy) noexcept;
Returns: (ymd.year() + dy) / ymd.month() / ymd.day().

Remarks: If ymd.month() is feb and ymd.day() is not in the range [1d, 28d], the resultant year_month_day may return false from ok().
constexpr year_month_day operator+(const years& dy, const year_month_day& ymd) noexcept;
Returns: ymd + dy.
constexpr year_month_day operator-(const year_month_day& ymd, const years& dy) noexcept;
Returns: ymd + (-dy).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month_day& ymd);
Effects: Inserts yyyy-mm-dd where the number of indicated digits are prefixed with '0' if necessary.

Returns: os.
template <class charT, class traits>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const year_month_day& ymd);
Effects: Streams ymd into os using the format specified by the null-terminated array fmt. fmt encoding follows the rules specified by [time.format].

Returns: os.
template <class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
            year_month_day& ymd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
            minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the year_month_day ymd using the format flags as specified in [time.parse].

If the parse fails to decode a valid year_month_day, ios_base::failbit will be set.

If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.

If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.

Returns: is.

23.17.10.14 Class year_month_day_last [time.calendar.year_month_day_last]

year_month_day_last represents a specific year, month, and the last day of the month. year_month_day_last is a field-based time point with a resolution of days, except that it is restricted to pointing to the last day of a year and month. One can observe each field. The day field is computed on demand. year_month_day_last supports years and months oriented arithmetic, but not days oriented arithmetic. For the latter, there is a conversion to sys_days which efficiently supports days oriented arithmetic. year_month_day_last is equality and less-than comparable.
class year_month_day_last
{
    chrono::year           y_;    // exposition only
    chrono::month_day_last mdl_;  // exposition only

public:
    constexpr year_month_day_last(const chrono::year& y,
                                  const chrono::month_day_last& mdl) noexcept;

    constexpr year_month_day_last& operator+=(const months& m) noexcept;
    constexpr year_month_day_last& operator-=(const months& m) noexcept;
    constexpr year_month_day_last& operator+=(const years& y)  noexcept;
    constexpr year_month_day_last& operator-=(const years& y)  noexcept;

    constexpr chrono::year           year()           const noexcept;
    constexpr chrono::month          month()          const noexcept;
    constexpr chrono::month_day_last month_day_last() const noexcept;
    constexpr chrono::day            day()            const noexcept;

    constexpr          operator sys_days()   const noexcept;
    constexpr explicit operator local_days() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator!=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator< (const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator> (const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator<=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
constexpr bool operator>=(const year_month_day_last& x, const year_month_day_last& y) noexcept;

constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const months& dm) noexcept;
constexpr year_month_day_last operator+(const months& dm, const year_month_day_last& ymdl) noexcept;
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const years& dy) noexcept;
constexpr year_month_day_last operator+(const years& dy, const year_month_day_last& ymdl) noexcept;
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const months& dm) noexcept;
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const years& dy) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_day_last& ymdl);
year_month_day_last is a trivially copyable class type.
year_month_day_last is a standard-layout class type.
constexpr year_month_day_last::year_month_day_last(const chrono::year& y,
                                                   const chrono::month_day_last& mdl) noexcept;
Effects: Constructs an object of type year_month_day_last by constructing y_ with y and mdl_ with mdl.
constexpr year_month_day_last& year_month_day_last::operator+=(const months& m) noexcept;
Effects: *this = *this + m;.

Returns: *this.
constexpr year_month_day_last& year_month_day_last::operator-=(const months& m) noexcept;
Effects: *this = *this - m;.

Returns: *this.
constexpr year_month_day_last& year_month_day_last::operator+=(const years& y) noexcept;
Effects: *this = *this + y;.

Returns: *this.
constexpr year_month_day_last& year_month_day_last::operator-=(const years& y) noexcept;
Effects: *this = *this - y;.

Returns: *this.
constexpr year year_month_day_last::year() const noexcept;
Returns: y_.
constexpr month year_month_day_last::month() const noexcept;
Returns: mdl_.month().
constexpr month_day_last year_month_day_last::month_day_last() const noexcept;
Returns: mdl_.
constexpr day year_month_day_last::day() const noexcept;
Returns: A day representing the last day of the year, month pair represented by *this.
constexpr year_month_day_last::operator sys_days() const noexcept;
Requires: ok() == true.

Effects: Equivalent to: return sys_days{year()/month()/day()};
constexpr explicit year_month_day_last::operator local_days() const noexcept;
Requires: ok() == true.

Effects: Equivalent to: return local_days{sys_days{*this}.time_since_epoch()};
constexpr bool year_month_day_last::ok() const noexcept;
Returns: y_.ok() && mdl_.ok().
constexpr bool operator==(const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: x.year() == y.year() && x.month_day_last() == y.month_day_last().
constexpr bool operator!=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: !(x == y).
constexpr bool operator< (const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: If x.year() < y.year(), returns true. Else if x.year() > y.year() returns false. Else returns x.month_day_last() < y.month_day_last().
constexpr bool operator> (const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: y < x.
constexpr bool operator<=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: !(y < x).
constexpr bool operator>=(const year_month_day_last& x, const year_month_day_last& y) noexcept;
Returns: !(x < y).
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const months& dm) noexcept;
Requires: ymdl.ok() is true.

Returns: (ymdl.year() / ymdl.month() + dm) / last.

Postconditions: The resultant year_month_day_last returns true from ok().

Complexity: O(1) with respect to the value of dm.
constexpr year_month_day_last operator+(const months& dm, const year_month_day_last& ymdl) noexcept;
Returns: ymdl + dm.
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const months& dm) noexcept;
Returns: ymdl + (-dm).
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const years& dy) noexcept;
Returns: {ymdl.year()+dy, ymdl.month_day_last()}.
constexpr year_month_day_last operator+(const years& dy, const year_month_day_last& ymdl) noexcept;
Returns: ymdl + dy.
constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const years& dy) noexcept;
Returns: ymdl + (-dy).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month_day_last& ymdl);
Effects: Equivalent to:
return os << ymdl.year() << '/' << ymdl.month_day_last();
23.17.10.15 Class year_month_weekday [time.calendar.year_month_weekday]

year_month_weekday represents a specific year, month, and nth weekday of the month. year_month_weekday is a field-based time point with a resolution of days. One can observe each field. year_month_weekday supports years and months oriented arithmetic, but not days oriented arithmetic. For the latter, there is a conversion to sys_days which efficiently supports days oriented arithmetic. year_month_weekday is equality comparable.
class year_month_weekday
{
    chrono::year            y_;    // exposition only
    chrono::month           m_;    // exposition only
    chrono::weekday_indexed wdi_;  // exposition only

public:
    year_month_weekday() = default;
    constexpr year_month_weekday(const chrono::year& y, const chrono::month& m,
                                 const chrono::weekday_indexed& wdi) noexcept;
    constexpr year_month_weekday(const sys_days& dp) noexcept;
    constexpr explicit year_month_weekday(const local_days& dp) noexcept;

    constexpr year_month_weekday& operator+=(const months& m) noexcept;
    constexpr year_month_weekday& operator-=(const months& m) noexcept;
    constexpr year_month_weekday& operator+=(const years& y)  noexcept;
    constexpr year_month_weekday& operator-=(const years& y)  noexcept;

    constexpr chrono::year            year()            const noexcept;
    constexpr chrono::month           month()           const noexcept;
    constexpr chrono::weekday         weekday()         const noexcept;
    constexpr unsigned                index()           const noexcept;
    constexpr chrono::weekday_indexed weekday_indexed() const noexcept;

    constexpr          operator sys_days()   const noexcept;
    constexpr explicit operator local_days() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr bool operator==(const year_month_weekday& x, const year_month_weekday& y) noexcept;
constexpr bool operator!=(const year_month_weekday& x, const year_month_weekday& y) noexcept;

constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;
constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const years& dy) noexcept;
constexpr year_month_weekday operator+(const years& dy, const year_month_weekday& ymwd) noexcept;
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const years& dy) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_weekday& ymwdi);
year_month_weekday is a trivially copyable class type.
year_month_weekday is a standard-layout class type.
constexpr year_month_weekday::year_month_weekday(const chrono::year& y, const chrono::month& m,
                                                 const chrono::weekday_indexed& wdi) noexcept;
Effects: Constructs an object of type year_month_weekday by constructing y_ with y, m_ with m, and wdi_ with wdi.
constexpr year_month_weekday(const sys_days& dp) noexcept;
Effects: Constructs an object of type year_month_weekday which corresponds to the date represented by dp.

Remarks: For any value of year_month_weekday, ymdl, for which ymdl.ok() is true, this equality will also be true: ymdl == year_month_weekday{sys_days{ymdl}}.
constexpr explicit year_month_weekday(const local_days& dp) noexcept;
Effects: Constructs an object of type year_month_weekday which corresponds to the date represented by dp.

Remarks: Equivalent to constructing with sys_days{dp.time_since_epoch()}.
constexpr year_month_weekday& year_month_weekday::operator+=(const months& m) noexcept;
Effects: *this = *this + m;.

Returns: *this.
constexpr year_month_weekday& year_month_weekday::operator-=(const months& m) noexcept;
Effects: *this = *this - m;.

Returns: *this.
constexpr year_month_weekday& year_month_weekday::operator+=(const years& y) noexcept;
Effects: *this = *this + y;.

Returns: *this.
constexpr year_month_weekday& year_month_weekday::operator-=(const years& y) noexcept;
Effects: *this = *this - y;.

Returns: *this.
constexpr year year_month_weekday::year() const noexcept;
Returns: y_.
constexpr month year_month_weekday::month() const noexcept;
Returns: m_.
constexpr weekday year_month_weekday::weekday() const noexcept;
Returns: wdi_.weekday().
constexpr unsigned year_month_weekday::index() const noexcept;
Returns: wdi_.index().
constexpr weekday_indexed year_month_weekday::weekday_indexed() const noexcept;
Returns: wdi_.
constexpr year_month_weekday::operator sys_days() const noexcept;
Requires: y_.ok() && m_.ok() && wdi_.weekday().ok() == true.

Returns: A sys_days which represents the date (index() - 1)*7 days after the first weekday() of year()/month(). If index() is 0 the returned sys_days represents the date 7 days prior to the first weekday() of year()/month().
constexpr explicit year_month_weekday::operator local_days() const noexcept;
Requires: y_.ok() && m_.ok() && wdi_.weekday().ok() == true.

Effects: Equivalent to: return local_days{sys_days{*this}.time_since_epoch()};
constexpr bool year_month_weekday::ok() const noexcept;
Returns: If y_.ok() or m_.ok() or wdi_.ok() returns false, returns false. Else if *this represents a valid date, returns true, else returns false.
constexpr bool operator==(const year_month_weekday& x, const year_month_weekday& y) noexcept;
Returns: x.year() == y.year() && x.month() == y.month() && x.weekday_indexed() == y.weekday_indexed().
constexpr bool operator!=(const year_month_weekday& x, const year_month_weekday& y) noexcept;
Returns: !(x == y).
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;
Requires: ymwd.ok() is true.

Returns: (ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed().

Postconditions: The resultant year_month_weekday returns true from ok().

Complexity: O(1) with respect to the value of dm.
constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;
Returns: ymwd + dm.
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;
Returns: ymwd + (-dm).
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const years& dy) noexcept;
Returns: {ymwd.year()+dy, ymwd.month(), ymwd.weekday_indexed()}.
constexpr year_month_weekday operator+(const years& dy, const year_month_weekday& ymwd) noexcept;
Returns: ymwd + dm.
constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const years& dy) noexcept;
Returns: ymwd + (-dm).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month_weekday& ymwd);
Effects: Equivalent to:
return os << ymwdi.year() << '/' << ymwdi.month() << '/' << ymwdi.weekday_indexed();
23.17.10.16 Class year_month_weekday_last [time.calendar.year_month_weekday_last]

year_month_weekday_last represents a specific year, month, and last weekday of the month. year_month_weekday_last is a field-based time point with a resolution of days, except that it is restricted to pointing to the last weekday of a year and month. One can observe each field. year_month_weekday_last supports years and months oriented arithmetic, but not days oriented arithmetic. For the latter, there is a conversion to sys_days which efficiently supports days oriented arithmetic. year_month_weekday_last is equality comparable.
class year_month_weekday_last
{
    chrono::year         y_;    // exposition only
    chrono::month        m_;    // exposition only
    chrono::weekday_last wdl_;  // exposition only

public:
    constexpr year_month_weekday_last(const chrono::year& y, const chrono::month& m,
                                      const chrono::weekday_last& wdl) noexcept;

    constexpr year_month_weekday_last& operator+=(const months& m) noexcept;
    constexpr year_month_weekday_last& operator-=(const months& m) noexcept;
    constexpr year_month_weekday_last& operator+=(const years& y)  noexcept;
    constexpr year_month_weekday_last& operator-=(const years& y)  noexcept;

    constexpr chrono::year         year()         const noexcept;
    constexpr chrono::month        month()        const noexcept;
    constexpr chrono::weekday      weekday()      const noexcept;
    constexpr chrono::weekday_last weekday_last() const noexcept;

    constexpr          operator sys_days()   const noexcept;
    constexpr explicit operator local_days() const noexcept;
    constexpr bool ok() const noexcept;
};

constexpr
bool
operator==(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;

constexpr
bool
operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;

constexpr
year_month_weekday_last
operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

constexpr
year_month_weekday_last
operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;

constexpr
year_month_weekday_last
operator+(const year_month_weekday_last& ymwdl, const years& dy) noexcept;

constexpr
year_month_weekday_last
operator+(const years& dy, const year_month_weekday_last& ymwdl) noexcept;

constexpr
year_month_weekday_last
operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;

constexpr
year_month_weekday_last
operator-(const year_month_weekday_last& ymwdl, const years& dy) noexcept;

template <class charT, class traits>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const year_month_weekday_last& ymwdl);
year_month_weekday_last is a trivially copyable class type.
year_month_weekday_last is a standard-layout class type.
constexpr year_month_weekday_last::year_month_weekday_last(const chrono::year& y, const chrono::month& m,
                                                           const chrono::weekday_last& wdl) noexcept;
Effects: Constructs an object of type year_month_weekday_last by constructing y_ with y, m_ with m, and wdl_ with wdl.
constexpr year_month_weekday_last& year_month_weekday_last::operator+=(const months& m) noexcept;
Effects: *this = *this + m;.

Returns: *this.
constexpr year_month_weekday_last& year_month_weekday_last::operator-=(const months& m) noexcept;
Effects: *this = *this - m;.

Returns: *this.
constexpr year_month_weekday_last& year_month_weekday_last::operator+=(const years& y) noexcept;
Effects: *this = *this + y;.

Returns: *this.
constexpr year_month_weekday_last& year_month_weekday_last::operator-=(const years& y) noexcept;
Effects: *this = *this - y;.

Returns: *this.
constexpr year year_month_weekday_last::year() const noexcept;
Returns: y_.
constexpr month year_month_weekday_last::month() const noexcept;
Returns: m_.
constexpr weekday year_month_weekday_last::weekday() const noexcept;
Returns: wdl_.weekday().
constexpr weekday_last year_month_weekday_last::weekday_last() const noexcept;
Returns: wdl_.
constexpr year_month_weekday_last::operator sys_days() const noexcept;
Requires: ok() == true.

Returns: A sys_days which represents the last weekday() of year()/month().
constexpr explicit year_month_weekday_last::operator local_days() const noexcept;
Requires: ok() == true.

Effects: Equivalent to: return local_days{sys_days{*this}.time_since_epoch()};
constexpr bool year_month_weekday_last::ok() const noexcept;
Returns: If y_.ok() && m_.ok() && wdl_.ok().
constexpr bool operator==(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;
Returns: x.year() == y.year() && x.month() == y.month() && x.weekday_last() == y.weekday_last().
constexpr bool operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) noexcept;
Returns: !(x == y).
constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
Requires: ymwdl.ok() is true.

Returns: (ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last().

Postconditions: The resultant year_month_weekday_last returns true from ok().

Complexity: O(1) with respect to the value of dm.
constexpr year_month_weekday_last operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;
Returns: ymwdl + dm.
constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
Returns: ymwdl + (-dm).
constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const years& dy) noexcept;
Returns: {ymwdl.year()+dy, ymwdl.month(), ymwdl.weekday_last()}.
constexpr year_month_weekday_last operator+(const years& dy, const year_month_weekday_last& ymwdl) noexcept;
Returns: ymwdl + dy.
constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const years& dy) noexcept;
Returns: ymwdl + (-dy).
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month_weekday_last& ymwdl);
Effects: Equivalent to:
return os << ymwdl.year() << '/' << ymwdl.month() << '/' << ymwdl.weekday_last();
23.17.10.17 civil calendar conventional syntax operators [time.calendar.operators]

A set of overloaded operator/() provide a conventional syntax for the creation of civil calendar dates. The year, month and day ordering are accepted in any of the following 3 orders:

y/m/d

m/d/y

d/m/y

Anywhere a "day" is required one can also specify one of:

last

weekday[i]

weekday[last]

Partial-date-types such as year_month and month_day can be created by simply not applying the second division operator for any of the three orders. For example:
year_month ym = 2015y/apr;
month_day md1 = apr/4;
month_day md2 = 4d/apr;
Everything not intended as above is ill-formed, with the notable exception of an expression that consists of nothing but int, which has type int.
auto a = 2015/4/4;       // a == int(125)
auto b = 2015y/4/4;      // b == year_month_day{year(2015), month(4), day(4)}
auto c = 2015y/4d/apr;   // error: invalid operands to binary expression ('chrono::year' and 'chrono::day')
auto d = 2015/apr/4;     // error: invalid operands to binary expression ('int' and 'const chrono::month')
year_month:
constexpr year_month operator/(const year& y, const month& m) noexcept;
Returns: {y, m}.
constexpr year_month operator/(const year& y, int   m) noexcept;
Returns: y / month(m).

month_day:
constexpr month_day operator/(const month& m, const day& d) noexcept;
Returns: {m, d}.
constexpr month_day operator/(const month& m, int d) noexcept;
Returns: m / day(d).
constexpr month_day operator/(int m, const day& d) noexcept;
Returns: month(m) / d.
constexpr month_day operator/(const day& d, const month& m) noexcept;
Returns: m / d.
constexpr month_day operator/(const day& d, int m) noexcept;
Returns: month(m) / d.

month_day_last:
constexpr month_day_last operator/(const month& m, last_spec) noexcept;
Returns: month_day_last{m}.
constexpr month_day_last operator/(int m, last_spec) noexcept;
Returns: month(m) / last.
constexpr month_day_last operator/(last_spec, const month& m) noexcept;
Returns: m / last.
constexpr month_day_last operator/(last_spec, int m) noexcept;
Returns: month(m) / last.

month_weekday:
constexpr month_weekday operator/(const month& m, const weekday_indexed& wdi) noexcept;
Returns: {m, wdi}.
constexpr month_weekday operator/(int m, const weekday_indexed& wdi) noexcept;
Returns: month(m) / wdi.
constexpr month_weekday operator/(const weekday_indexed& wdi, const month& m) noexcept;
Returns: m / wdi.
constexpr month_weekday operator/(const weekday_indexed& wdi, int m) noexcept;
Returns: month(m) / wdi.

month_weekday_last:
constexpr month_weekday_last operator/(const month& m, const weekday_last& wdl) noexcept;
Returns: {m, wdl}.
constexpr month_weekday_last operator/(int m, const weekday_last& wdl) noexcept;
Returns: month(m) / wdl.
constexpr month_weekday_last operator/(const weekday_last& wdl, const month& m) noexcept;
Returns: m / wdl.
constexpr month_weekday_last operator/(const weekday_last& wdl, int m) noexcept;
Returns: month(m) / wdl.

year_month_day:
constexpr year_month_day operator/(const year_month& ym, const day& d) noexcept;
Returns: {ym.year(), ym.month(), d}.
constexpr year_month_day operator/(const year_month& ym, int d) noexcept;
Returns: ym / day(d).
constexpr year_month_day operator/(const year& y, const month_day& md) noexcept;
Returns: y / md.month() / md.day().
constexpr year_month_day operator/(int y, const month_day& md) noexcept;
Returns: year(y) / md.
constexpr year_month_day operator/(const month_day& md, const year& y) noexcept;
Returns: y / md.
constexpr year_month_day operator/(const month_day& md, int y) noexcept;
Returns: year(y) / md.

year_month_day_last:
constexpr year_month_day_last operator/(const year_month& ym, last_spec) noexcept;
Returns: {ym.year(), month_day_last{ym.month()}}.
constexpr year_month_day_last operator/(const year& y, const month_day_last& mdl) noexcept;
Returns: {y, mdl}.
constexpr year_month_day_last operator/(int y, const month_day_last& mdl) noexcept;
Returns: year(y) / mdl.
constexpr year_month_day_last operator/(const month_day_last& mdl, const year& y) noexcept;
Returns: y / mdl.
constexpr year_month_day_last operator/(const month_day_last& mdl, int y) noexcept;
Returns: year(y) / mdl.

year_month_weekday:
constexpr year_month_weekday operator/(const year_month& ym, const weekday_indexed& wdi) noexcept;
Returns: {ym.year(), ym.month(), wdi}.
constexpr year_month_weekday operator/(const year& y, const month_weekday& mwd) noexcept;
Returns: {y, mwd.month(), mwd.weekday_indexed()}.
constexpr year_month_weekday operator/(int y, const month_weekday& mwd) noexcept;
Returns: year(y) / mwd.
constexpr year_month_weekday operator/(const month_weekday& mwd, const year& y) noexcept;
Returns: y / mwd.
constexpr year_month_weekday operator/(const month_weekday& mwd, int y) noexcept;
Returns: year(y) / mwd.

year_month_weekday_last:
constexpr year_month_weekday_last operator/(const year_month& ym, const weekday_last& wdl) noexcept;
Returns: {ym.year(), ym.month(), wdl}.
constexpr year_month_weekday_last operator/(const year& y, const month_weekday_last& mwdl) noexcept;
Returns: {y, mwdl.month(), mwdl.weekday_last()}.
constexpr year_month_weekday_last operator/(int y, const month_weekday_last& mwdl) noexcept;
Returns: year(y) / mwdl.
constexpr year_month_weekday_last operator/(const month_weekday_last& mwdl, const year& y) noexcept;
Returns: y / mwdl.
constexpr year_month_weekday_last operator/(const month_weekday_last& mwdl, int y) noexcept;
Returns: year(y) / mwdl.

Back to TOC

Add new section [time.time_of_day] after 23.17.10 The civil calendar [time.calendar]:

23.17.11 Class time_of_day [time.time_of_day]

The time_of_day class breaks a duration which represents the time elapsed since midnight, into a "broken" down time such as hours:minutes:seconds. The Duration template parameter dictates the precision to which the time is broken down. This can vary from a course precision of hours to a very fine precision of nanoseconds. time_of_day is primarily a formatting tool.
template <class Duration> class time_of_day;
There are 4 specializations of time_of_day to handle four precisions:
tempalte <> class time_of_day<hours>
This specialization handles hours since midnight.
tempalte <> class time_of_day<minutes>
This specialization handles hours:minutes since midnight.
tempalte <> class time_of_day<seconds>
This specialization handles hours:minutes:seconds since midnight.
tempalte <class Rep, class Period> class time_of_day<duration<Rep, Period>>
This specialization is restricted to Rep types that are integral, and Periods that are not an integral number of seconds. Typical uses are with milliseconds, microseconds and nanoseconds. This specialization handles hours:minute:seconds.fractional_seconds since midnight.
Each specialization of time_of_day is a trivially copyable class type.
Each specialization of time_of_day is a standard-layout class type.
tempalte <>
class time_of_day<hours>
{
public:
    using precision = hours;

    time_of_day() = default;
    constexpr explicit time_of_day(hours since_midnight) noexcept;

    constexpr hours    hours() const noexcept;

    constexpr explicit operator  precision()   const noexcept;
    constexpr          precision to_duration() const noexcept;

    constexpr void make24() noexcept;
    constexpr void make12() noexcept;
};
constexpr explicit time_of_day<hours>::time_of_day(hours since_midnight) noexcept;
Effects: Constructs an object of type time_of_day in 24-hour format corresponding to since_midnight hours after 00:00:00.

Postconditions: hours() returns the integral number of hours since_midnight is after 00:00:00.
constexpr hours time_of_day<hours>::hours() const noexcept;
Returns: The stored hour of *this.
constexpr explicit time_of_day<hours>::operator precision() const noexcept;
Returns: The number of hours since midnight.
constexpr precision to_duration() const noexcept;
Returns: precision{*this}.
constexpr void time_of_day<hours>::make24() noexcept;
Effects: If *this is a 12-hour time, converts to a 24-hour time. Otherwise, no effects.
constexpr void time_of_day<hours>::make12() noexcept;
Effects: If *this is a 24-hour time, converts to a 12-hour time. Otherwise, no effects.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<hours>& t);
Effects: If t is a 24-hour time, outputs to os according to the strftime format: "%H00". "%H" will emit a leading 0 for hours less than 10. Else t is a 12-hour time, outputs to os according to the strftime format: "%I%p" according to the C locale, except that no leading zero is output for hours less than 10.

Returns: os.

Example:
0100  // 1 in the morning in 24-hour format
1800  // 6 in the evening in 24-hour format
1am   // 1 in the morning in 12-hour format
6pm   // 6 in the evening in 12-hour format
tempalte <>
class time_of_day<minutes>
{
public:
    using precision = minutes;

    time_of_day() = default;
    constexpr explicit time_of_day(minutes since_midnight) noexcept;

    constexpr hours    hours()   const noexcept;
    constexpr minutes  minutes() const noexcept;

    constexpr explicit operator precision()    const noexcept;
    constexpr          precision to_duration() const noexcept;

    void make24() noexcept;
    void make12() noexcept;
};
constexpr explicit time_of_day<minutes>::time_of_day(minutes since_midnight) noexcept;
Effects: Constructs an object of type time_of_day in 24-hour format corresponding to since_midnight minutes after 00:00:00.

Postconditions: hours() returns the integral number of hours since_midnight is after 00:00:00. minutes() returns the integral number of minutes since_midnight is after (00:00:00 + hours()).
constexpr hours time_of_day<minutes>::hours() const noexcept;
Returns: The stored hour of *this.
constexpr minutes time_of_day<minutes>::minutes() const noexcept;
Returns: The stored minute of *this.
constexpr explicit time_of_day<minutes>::operator precision() const noexcept;
Returns: The number of minutes since midnight.
constexpr precision to_duration() const noexcept;
Returns: precision{*this}.
void time_of_day<minutes>::make24() noexcept;
Effects: If *this is a 12-hour time, converts to a 24-hour time. Otherwise, no effects.
void time_of_day<minutes>::make12() noexcept;
Effects: If *this is a 24-hour time, converts to a 12-hour time. Otherwise, no effects.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<minutes>& t);
Effects: If t is a 24-hour time, outputs to os according to the strftime format: "%H:%M". "%H" will emit a leading 0 for hours less than 10. Else t is a 12-hour time, outputs to os according to the strftime format: "%I:%M%p" according to the C locale, except that no leading zero is output for hours less than 10.

Returns: os.

Example:
01:08   // 1:08 in the morning in 24-hour format
18:15   // 6:15 in the evening in 24-hour format
1:08am  // 1:08 in the morning in 12-hour format
6:15pm  // 6:15 in the evening in 12-hour format
tempalte <>
class time_of_day<seconds>
{
public:
    using precision = seconds;

    time_of_day() = default;
    constexpr explicit time_of_day(seconds since_midnight) noexcept;

    constexpr hours    hours()   const noexcept;
    constexpr minutes  minutes() const noexcept;
    constexpr seconds  seconds() const noexcept;

    constexpr explicit operator  precision()   const noexcept;
    constexpr          precision to_duration() const noexcept;

    void make24() noexcept;
    void make12() noexcept;
};
constexpr explicit time_of_day<seconds>::time_of_day(seconds since_midnight) noexcept;
Effects: Constructs an object of type time_of_day in 24-hour format corresponding to since_midnight seconds after 00:00:00.

Postconditions: hours() returns the integral number of hours since_midnight is after 00:00:00. minutes() returns the integral number of minutes since_midnight is after (00:00:00 + hours()). seconds() returns the integral number of seconds since_midnight is after (00:00:00 + hours() + minutes()).
constexpr hours time_of_day<seconds>::hours() const noexcept;
Returns: The stored hour of *this.
constexpr minutes time_of_day<seconds>::minutes() const noexcept;
Returns: The stored minute of *this.
constexpr seconds time_of_day<seconds>::seconds() const noexcept;
Returns: The stored second of *this.
constexpr explicit time_of_day<seconds>::operator precision() const noexcept;
Returns: The number of seconds since midnight.
constexpr precision to_duration() const noexcept;
Returns: precision{*this}.
void time_of_day<seconds>::make24() noexcept;
Effects: If *this is a 12-hour time, converts to a 24-hour time. Otherwise, no effects.
void time_of_day<seconds>::make12() noexcept;
Effects: If *this is a 24-hour time, converts to a 12-hour time. Otherwise, no effects.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<seconds>& t);
Effects: If t is a 24-hour time, outputs to os according to the strftime format: "%H:%M%S". "%H" will emit a leading 0 for hours less than 10. Else t is a 12-hour time, outputs to os according to the strftime format: "%I:%M%S%p" according to the C locale, except that no leading zero is output for hours less than 10.

Returns: os.

Example:
01:08:03   // 1:08:03 in the morning in 24-hour format
18:15:45   // 6:15:45 in the evening in 24-hour format
1:08:03am  // 1:08:03 in the morning in 12-hour format
6:15:45pm  // 6:15:45 in the evening in 12-hour format
tempalte <class Rep, class Period>
class time_of_day<duration<Rep, Period>>
{
public:
    using precision = duration<Rep, Period>;

    time_of_day() = default;
    constexpr explicit time_of_day(precision since_midnight) noexcept;

    constexpr hours     hours()      const noexcept;
    constexpr minutes   minutes()    const noexcept;
    constexpr seconds   seconds()    const noexcept;
    constexpr precision subseconds() const noexcept;

    constexpr explicit operator  precision()   const noexcept;
    constexpr          precision to_duration() const noexcept;

    void make24() noexcept;
    void make12() noexcept;
};
This specialization shall not exist unless treat_as_floating_point<Rep>::value is false and duration<Rep, Period> is not convertible to seconds.
constexpr explicit time_of_day<duration<Rep, Period>>::time_of_day(precision since_midnight) noexcept;
Effects: Constructs an object of type time_of_day in 24-hour format corresponding to since_midnight precision fractional seconds after 00:00:00.

Postconditions: hours() returns the integral number of hours since_midnight is after 00:00:00. minutes() returns the integral number of minutes since_midnight is after (00:00:00 + hours()). seconds() returns the integral number of seconds since_midnight is after (00:00:00 + hours() + minutes()). subseconds() returns the integral number of fractional precision seconds since_midnight is after (00:00:00 + hours() + minutes() + seconds).
constexpr hours time_of_day<duration<Rep, Period>>::hours() const noexcept;
Returns: The stored hour of *this.
constexpr minutes time_of_day<duration<Rep, Period>>::minutes() const noexcept;
Returns: The stored minute of *this.
constexpr seconds time_of_day<duration<Rep, Period>>::seconds() const noexcept;
Returns: The stored second of *this.
constexpr
duration<Rep, Period>
time_of_day<duration<Rep, Period>>::subseconds() const noexcept;
Returns: The stored subsecond of *this.
constexpr explicit time_of_day<duration<Rep, Period>>::operator precision() const noexcept;
Returns: The number of subseconds since midnight.
constexpr precision to_duration() const noexcept;
Returns: precision{*this}.
void time_of_day<duration<Rep, Period>>::make24() noexcept;
Effects: If *this is a 12-hour time, converts to a 24-hour time. Otherwise, no effects.
void time_of_day<duration<Rep, Period>>::make12() noexcept;
Effects: If *this is a 24-hour time, converts to a 12-hour time. Otherwise, no effects.
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const time_of_day<duration<Rep, Period>>& t);
Effects: If t is a 24-hour time, outputs to os according to the strftime format: "%H:%M%S.%s". "%H" will emit a leading 0 for hours less than 10. "%s" is not a strftime code and represents the fractional seconds. Else t is a 12-hour time, outputs to os according to the strftime format: "%I:%M%S.%s%p" according to the C locale, except that no leading zero is output for hours less than 10.

Returns: os.

Example:
01:08:03.007   // 1:08:03.007 in the morning in 24-hour format (assuming millisecond precision)
18:15:45.123   // 6:15:45.123 in the evening in 24-hour format (assuming millisecond precision)
1:08:03.007am  // 1:08:03.007 in the morning in 12-hour format (assuming millisecond precision)
6:15:45.123pm  // 6:15:45.123 in the evening in 12-hour format (assuming millisecond precision)
Back to TOC

Add new section [time.timezone] after 23.17.11 Class time_of_day [time.time_of_day]:

23.17.12 Time Zones [time.timezone]

This clause creates an API which exposes the IANA Time Zone database, and interfaces with sys_time and local_time. By using only this interface, time zone support is provided not only to the civil calendar types, but also to other user-written calendars that interface with sys_time and local_time.

23.17.12.1 The time zone database [time.timezone.database]

The following data structure is the time zone database, and the following functions access it.
struct tzdb
{
    string            version;
    vector<time_zone> zones;
    vector<link>      links;
    vector<leap>      leaps;

    const time_zone* locate_zone(string_view tz_name) const;
    const time_zone* current_zone() const;
};

class tzdb_list
{
    std::atomic<tzdb*> head_{nullptr};  // exposition only

public:
    class const_iterator;

    const tzdb& front() const noexcept;

    const_iterator erase_after(const_iterator p) noexcept;

    const_iterator begin() const noexcept;
    const_iterator end()   const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cend()   const noexcept;
};
The tzdb_list database is a singleton. Access is granted to it via the get_tzdb_list() function which returns a reference to it. However this access is only needed for those applications which need to have long uptimes and have a need to update the time zone database while running. Other applications can implicitly access the front() of this list via the read-only namespace scope functions get_tzdb(), locate_zone() and current_zone(). Each vector in tzdb is sorted to enable fast lookup. One can iterate over and inspect this database. And multiple versions of the database can be used at once, via the tzdb_list.
const time_zone* tzdb::locate_zone(string_view tz_name) const;
Returns: If a time_zone is found for which name() == tz_name, returns a pointer to that time_zone. Otherwise if a link is found where tz_name == link.name(), then a pointer is returned to the time_zone for which zone.name() == link.target() [Note: A link is an alternative name for a time_zone. — end note]

Throws: If a const time_zone* can not be found as described in the Returns clause, throws a runtime_error. [Note: On non-exceptional return, the return value is always a pointer to a valid time_zone. — end note]
const time_zone* tzdb::current_zone() const;
Returns: A const time_zone* referring to the time zone which your computer has set as its local time zone.
list<tzdb>& get_tzdb_list();
Effects: If this is the first access to the database, will initialize the database. If this call initializes the database, the resulting database will be a tzdb_list which holds a single initialized tzdb.

Returns: A reference to the database.

Thread Safety: It is safe to call this function from multiple threads at one time.

Throws: runtime_error if for any reason a reference can not be returned to a valid list<tzdb>& containing one or more valid tzdb.
const tzdb& get_tzdb();
Returns: get_tzdb_list().front().
const time_zone* locate_zone(string_view tz_name);
Returns: get_tzdb().locate_zone(tz_name) which will initialize the timezone database if this is the first reference to the database.
const time_zone* current_zone();
Returns: get_tzdb().current_zone().

tzdb_list::const_iterator is a constant iterator which meets the forward iterator requirements and has a value type of tzdb.
const tzdb& tzdb_list::front() const noexcept;
Returns: *head_.

Remarks: this operation is thread safe with respect to reload_tzdb(). [Note: reload_tzdb() pushes a new tzdb onto the front of this container. — end note]
tzdb::const_iterator tzdb::erase_after(const_iterator p) noexcept;
Requires: The iterator following p is dereferenceable.

Effects: Erases the tzdb referred to by the iterator following p.

Returns: An iterator pointing to the element following the one that was erased, or end() if no such element exists.

Remarks: No pointers, references or iterators are invalidated except those referring to the erased tzdb.

Note: It is not possible to erase the tzdb referred to by begin().
tzdb::const_iterator tzdb::begin() const noexcept;
Returns: An iterator referring to the first tzdb in the container.
tzdb::const_iterator tzdb::end() const noexcept;
Returns: An iterator referring to the position one past the last tzdb in the container.
tzdb::const_iterator tzdb::cbegin() const noexcept;
Returns: begin().
tzdb::const_iterator tzdb::cend() const noexcept;
Returns: end().

Back to TOC

23.17.12.1.1 Remote time zone database support [time.timezone.database.remote]

The local time zone database is that supplied by the implementation when the application first accesses the database, for example via current_zone(). While the application is running, the implementation may choose to update the time zone database. This update shall not impact the application in any way unless the application calls the functions in this section. This potentially updated time zone database is referred to as the remote time zone database.
const tzdb& reload_tzdb();
Effects: This function first checks the version of the remote time zone database. If the version of the local and remote databases are the same, there are no effects. Otherwise the remote database is pushed to the front of the tzdb_list accessed by get_tzdb_list().

Returns: get_tzdb_list().front().

Remarks: No pointers, references or iterators are invalidated.

Thread Safety: This function is thread safe with respect to get_tzdb_list().front() and get_tzdb_list().erase_after().

Throws: runtime_error if for any reason a reference can not be returned to a valid tzdb.
string remote_version();
Returns: The latest remote database version. If the remote version s not available, the empty string is returned.

Note: If non-empty, this can be compared with get_tzdb().version to discover if the local and remote databases are equivalent.

23.17.12.2 Exception classes [time.timezone.exception]

nonexistent_local_time is thrown when one attempts to convert a non-existent local_time to a sys_time without specifying choose::earliest or choose::latest.
class nonexistent_local_time
    : public runtime_error
{
public:
    // Construction is undocumented
};
[Example:
#include "tz.h"
#include <iostream>

int
main()
{
    using namespace std::chrono;
    try
    {
        auto zt = zoned_time{"America/New_York", local_days{sun[2]/mar/2016} + 2h + 30min};
    }
    catch (const nonexistent_local_time& e)
    {
        std::cout << e.what() << '\n';
    }
}
Which outputs:
2016-03-13 02:30:00 is in a gap between
2016-03-13 02:00:00 EST and
2016-03-13 03:00:00 EDT which are both equivalent to
2016-03-13 07:00:00 UTC
— end example:]

ambiguous_local_time is thrown when one attempts to convert an ambiguous local_time to a sys_time without specifying choose::earliest or choose::latest.
class ambiguous_local_time
    : public runtime_error
{
public:
    // Construction is undocumented
};
[Example:
#include "tz.h"
#include <iostream>

int
main()
{
    using namespace std::chrono;
    try
    {
        auto zt = zoned_time{"America/New_York", local_days{sun[1]/nov/2016} + 1h + 30min};
    }
    catch (const ambiguous_local_time& e)
    {
        std::cout << e.what() << '\n';
    }
}
Which outputs:
2016-11-06 01:30:00 is ambiguous.  It could be
2016-11-06 01:30:00 EDT == 2016-11-06 05:30:00 UTC or
2016-11-06 01:30:00 EST == 2016-11-06 06:30:00 UTC
— end example:]

23.17.12.3 Information classes [time.timezone.info]

A sys_info structure can be obtained from the combination of a time_zone and either a sys_time, or local_time. It can also be obtained from a zoned_time which is effectively a pair of a time_zone and sys_time.

This structure represents a lower-level API. Typical conversions from sys_time to local_time will use this structure implicitly, not explicitly.
struct sys_info
{
    sys_seconds   begin;
    sys_seconds   end;
    seconds       offset;
    minutes       save;
    string        abbrev;
};
The begin and end fields indicate that for the associated time_zone and time_point, the offset and abbrev are in effect in the range [begin, end). This information can be used to efficiently iterate the transitions of a time_zone.

The offset field indicates the UTC offset in effect for the associated time_zone and time_point. The relationship between local_time and sys_time is:
offset = local_time - sys_time
The save field is "extra" information not normally needed for conversion between local_time and sys_time. If save != 0min, this sys_info is said to be on "daylight saving" time, and offset - save suggests what this time_zone might use if it were off daylight saving. However this information should not be taken as authoritative. The only sure way to get such information is to query the time_zone with a time_point that returns an sys_info where save == 0min. There is no guarantee what time_point might return such an sys_info except that it is guaranteed not to be in the range [begin, end) (if save != 0min for this sys_info).

The abbrev field indicates the current abbreviation used for the associated time_zone and time_point. Abbreviations are not unique among the time_zones, and so one can not reliably map abbreviations back to a time_zone and UTC offset.

A sys_info can be streamed out in an unspecified format:
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const sys_info& r);
A local_info structure represents a lower-level API. Typical conversions from local_time to sys_time will use this structure implicitly, not explicitly.
struct local_info
{
    enum {unique, nonexistent, ambiguous} result;
    sys_info first;
    sys_info second;
};
When a local_time to sys_time conversion is unique, result == unique, first will be filled out with the correct sys_info and second will be zero-initialized. If the conversion stems from a nonexistent local_time then result == nonexistent, first will be filled out with the sys_info that ends just prior to the local_time and second will be filled out with the sys_info that begins just after the local_time. If the conversion stems from an ambiguous local_time then result == ambiguous, first will be filled out with the sys_info that ends just after the local_time and second will be filled out with the sys_info that starts just before the local_time.

A local_info can be streamed out in an unspecified format:
template <class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const local_info& r);
23.17.12.4 Class time_zone [time.timezone.time_zone]

A time_zone represents all time zone transitions for a specific geographic area. time_zone construction is undocumented, and done during the database initialization. You can gain const access to a time_zone via functions such as locate_zone.
class time_zone
{
public:
    time_zone(const time_zone&) = delete;
    time_zone& operator=(const time_zone&) = delete;

    const string& name() const noexcept;

    template <class Duration> sys_info   get_info(sys_time<Duration> st)   const;
    template <class Duration> local_info get_info(local_time<Duration> tp) const;

    template <class Duration>
        sys_time<typename common_type<Duration, seconds>::type>
        to_sys(local_time<Duration> tp) const;

    template <class Duration>
        sys_time<typename common_type<Duration, seconds>::type>
        to_sys(local_time<Duration> tp, choose z) const;

    template <class Duration>
        local_time<typename common_type<Duration, seconds>::type>
        to_local(sys_time<Duration> tp) const;
};

bool operator==(const time_zone& x, const time_zone& y) noexcept;
bool operator!=(const time_zone& x, const time_zone& y) noexcept;
bool operator< (const time_zone& x, const time_zone& y) noexcept;
bool operator> (const time_zone& x, const time_zone& y) noexcept;
bool operator<=(const time_zone& x, const time_zone& y) noexcept;
bool operator>=(const time_zone& x, const time_zone& y) noexcept;
const string& time_zone::name() const noexcept;
Returns: The name of the time_zone.

Example: "America/New_York".

Note: Here is an unofficial list of time_zone names: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones.
template <class Duration> sys_info time_zone::get_info(sys_time<Duration> st) const;
Returns: A sys_info i for which st is in the range [i.begin, i.end).
template <class Duration> local_info time_zone::get_info(local_time<Duration> tp) const;
Returns: A local_info for tp.
template <class Duration>
sys_time<typename common_type<Duration, seconds>::type>
time_zone::to_sys(local_time<Duration> tp) const;
Returns: A sys_time that is at least as fine as seconds, and will be finer if the argument tp has finer precision. This sys_time is the UTC equivalent of tp according to the rules of this time_zone.

Throws: If the conversion from tp to a sys_time is ambiguous, throws ambiguous_local_time. If the conversion from tp to a sys_time is nonexistent, throws nonexistent_local_time.
template <class Duration>
sys_time<typename common_type<Duration, seconds>::type>
time_zone::to_sys(local_time<Duration> tp, choose z) const;
Returns: A sys_time that is at least as fine as seconds, and will be finer if the argument tp has finer precision. This sys_time is the UTC equivalent of tp according to the rules of this time_zone. If the conversion from tp to a sys_time is ambiguous, returns the earlier sys_time if z == choose::earliest, and returns the later sys_time if z == choose::latest. If the tp represents a non-existent time between two UTC time_points, then the two UTC time_points will be the same, and that UTC time_point will be returned.
template <class Duration>
local_time<typename common_type<Duration, seconds>::type>
time_zone::to_local(sys_time<Duration> tp) const;
Returns: The local_time associated with tp and this time_zone.
bool operator==(const time_zone& x, const time_zone& y) noexcept;
Returns: x.name() == y.name().
bool operator!=(const time_zone& x, const time_zone& y) noexcept;
Returns: !(x == y).
bool operator<(const time_zone& x, const time_zone& y) noexcept;
Returns: x.name() < y.name().
bool operator>(const time_zone& x, const time_zone& y) noexcept;
Returns: y < x.
bool operator<=(const time_zone& x, const time_zone& y) noexcept;
Returns: !(y < x).
bool operator>=(const time_zone& x, const time_zone& y) noexcept;
Returns: !(x < y).

23.17.12.5 Class zoned_traits [time.timezone.zoned_traits]

zoned_traits provides a means for customizing the behavior of zoned_time<Duration, TimeZonePtr> for the zoned_time default constructor, and constructors taking string_view. A specialization for const time_zone* is provided by the implementation.
template <class T> struct zoned_traits {};

template <>
struct zoned_traits<const time_zone*>
{
    static const time_zone* default_zone();
    static const time_zone* locate_zone(string_view name);
};
static const time_zone* zoned_traits<const time_zone*>::default_zone();
Returns: std::chrono::locate_zone("UTC").
static const time_zone* zoned_traits<const time_zone*>::locate_zone(string_view name);
Returns: std::chrono::locate_zone(name).

23.17.12.6 Class zoned_time [time.timezone.zoned_time]

zoned_time represents a logical paring of time_zone and a time_point with precision Duration.
template <class Duration, class TimeZonePtr = const time_zone*>
class zoned_time
{
public:
    using duration = common_type_t<Duration, seconds>;

private:
    TimeZonePtr        zone_;  // exposition only
    sys_time<duration> tp_;    // exposition only

public:
    zoned_time();
    zoned_time(const zoned_time&) = default;
    zoned_time& operator=(const zoned_time&) = default;

             zoned_time(const sys_time<Duration>& st);
    explicit zoned_time(TimeZonePtr z);
    explicit zoned_time(string_view name);

    template <class Duration2>
        zoned_time(const zoned_time<Duration2>& zt) noexcept;

    zoned_time(TimeZonePtr z,    const sys_time<Duration>& st);
    zoned_time(string_view name, const sys_time<Duration>& st);

    zoned_time(TimeZonePtr z,    const local_time<Duration>& tp);
    zoned_time(string_view name, const local_time<Duration>& tp);
    zoned_time(TimeZonePtr z,    const local_time<Duration>& tp, choose c);
    zoned_time(string_view name, const local_time<Duration>& tp, choose c);

    zoned_time(TimeZonePtr z,    const zoned_time<Duration>& zt);
    zoned_time(string_view name, const zoned_time<Duration>& zt);
    zoned_time(TimeZonePtr z,    const zoned_time<Duration>& zt, choose);
    zoned_time(string_view name, const zoned_time<Duration>& zt, choose);

    zoned_time& operator=(const sys_time<Duration>& st);
    zoned_time& operator=(const local_time<Duration>& ut);

             operator sys_time<duration>()   const;
    explicit operator local_time<duration>() const;

    TimeZonePtr          get_time_zone()  const;
    local_time<duration> get_local_time() const;
    sys_time<duration>   get_sys_time()   const;
    sys_info             get_info()       const;
};

template <class Duration1, class Duration2, class TimeZonePtr>
bool
operator==(const zoned_time<Duration1, TimeZonePtr>& x,
           const zoned_time<Duration2, TimeZonePtr>& y);

template <class Duration1, class Duration2, class TimeZonePtr>
bool
operator!=(const zoned_time<Duration1, TimeZonePtr>& x,
           const zoned_time<Duration2, TimeZonePtr>& y);

template <class charT, class traits, class Duration, class TimeZonePtr>
    basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os,
               const zoned_time<Duration, TimeZonePtr>& t);

template <class charT, class traits, class Duration, class TimeZonePtr>
    basic_ostream<charT, traits>&
    to_stream(basic_ostream<charT, traits>& os, const charT* fmt,
              const zoned_time<Duration, TimeZonePtr>& tp);

zoned_time()
    -> zoned_time<seconds>;

template <class Duration>
    zoned_time(sys_time<Duration>)
        -> zoned_time<common_type_t<Duration, seconds>>;

template <class TimeZonePtr, class Duration>
    zoned_time(TimeZonePtr, sys_time<Duration>)
        -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

template <class TimeZonePtr, class Duration>
    zoned_time(TimeZonePtr, local_time<Duration>, choose = choose::earliest)
        -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

template <class TimeZonePtr, class Duration>
    zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest)
        -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

zoned_time(string_view)
    -> zoned_time<seconds>;

template <class Duration>
zoned_time(string_view, sys_time<Duration>)
    -> zoned_time<common_type_t<Duration, seconds>>;

template <class Duration>
zoned_time(string_view, local_time<Duration>, choose = choose::earliest)
    -> zoned_time<common_type_t<Duration, seconds>>;
An invariant of zoned_time<Duration> is that it always refers to a valid time_zone, and represents a point in time that exists and is not ambiguous.
zoned_time<Duration, TimeZonePtr>::zoned_time();
Remarks: This constructor does not participate in overload resolution unless the expression zoned_traits<TimeZonePtr>::default_zone() is well formed.

Effects: Constructs a zoned_time by initializing zone_ with zoned_traits<TimeZonePtr>::default_zone() and default constructing tp_.
zoned_time<Duration, TimeZonePtr>::zoned_time(const zoned_time&) = default;
zoned_time<Duration, TimeZonePtr>& zoned_time<Duration>::operator=(const zoned_time&) = default;
The copy members transfer the associated time_zone from the source to the destination. After copying, source and destination compare equal. If Duration has noexcept copy members, then zoned_time<Duration> has noexcept copy members.
zoned_time<Duration, TimeZonePtr>::zoned_time(const sys_time<Duration>& st);
Remarks: This constructor does not participate in overload resolution unless the expression zoned_traits<TimeZonePtr>::default_zone() is well formed.

Effects: Constructs a zoned_time by initializing zone_ with zoned_traits<TimeZonePtr>::default_zone() and tp_ with st.
explicit zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z);
Requires: z refers to a valid time_zone.

Effects: Constructs a zoned_time initializing zone_ with std::move(z).
explicit zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name);
Remarks: This constructor does not participate in overload resolution unless the expression zoned_traits<TimeZonePtr>::locate_zone(string_view{}) is well formed and zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(string_view{}).

Effects: Constructs a zoned_time by initializing zone_ with zoned_traits<TimeZonePtr>::locate_zone(name) and default constructing tp_.
template <class Duration2, TimeZonePtr>
    zoned_time<Duration>::zoned_time(const zoned_time<Duration2, TimeZonePtr>& y) noexcept;
Remarks: Does not participate in overload resolution unless sys_time<Duration2> is implicitly convertible to sys_time<Duration>.

Effects: Constructs a zoned_time x such that x == y.
zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z, const sys_time<Duration>& st);
Requires: z refers to a valid time_zone.

Effects: Constructs a zoned_time by initializing zone_ with std::move(z) and tp_ with st.
zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name, const sys_time<Duration>& st);
Remarks: This constructor does not participate in overload resolution unless zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(name) and st.

Effects: Equivalent to construction with {zoned_traits<TimeZonePtr>::locate_zone(name), st}.
zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z, const local_time<Duration>& tp);
Requires: z refers to a valid time_zone.

Remarks: This constructor does not participate in overload resolution unless declval<TimeZonePtr&>()->to_sys(local_time<Duration>{}) is convertible to sys_time<duration>.

Effects: Constructs a zoned_time by initializing zone_ with std::move(z) and tp_ with zone_->to_sys(t).
zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name, const local_time<Duration>& tp);
Remarks: This constructor does not participate in overload resolution unless zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(name) and tp.

Effects: Equivalent to construction with {zoned_traits<TimeZonePtr>::locate_zone(name), tp}.
zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z, const local_time<Duration>& tp, choose c);
Requires: z refers to a valid time_zone.

Remarks: This constructor does not participate in overload resolution unless decltype(declval<TimeZonePtr&>()->to_sys(local_time<Duration>{}, choose::earliest)) is convertible to sys_time<duration>.

Effects: Constructs a zoned_time by initializing zone_ with std::move(z) and tp_ with zone_->to_sys(t, c).
zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name, const local_time<Duration>& tp, choose c);
Remarks: This constructor does not participate in overload resolution unless zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(name), local_time<Duration> and choose.

Effects: Equivalent to construction with {zoned_traits<TimeZonePtr>::locate_zone(name), tp, c}.
zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z, const zoned_time<Duration>& y);
Requires: z refers to a valid time_zone.

Effects: Constructs a zoned_time by initializing zone_ with std::move(z) and tp_ with z.tp_.
zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name, const zoned_time<Duration>& y);
Remarks: This constructor does not participate in overload resolution unless zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(name) and zoned_time.

Effects: Equivalent to construction with {zoned_traits<TimeZonePtr>::locate_zone(name), y}.
zoned_time<Duration, TimeZonePtr>::zoned_time(TimeZonePtr z, const zoned_time<Duration>& y, choose);
Requires: z refers to a valid time_zone.

Effects: Equivalent to construction with {z, y}.

Note: The choose parameter is allowed here, but has no impact.
zoned_time<Duration, TimeZonePtr>::zoned_time(string_view name, const zoned_time<Duration>& y, choose c);
Remarks: This constructor does not participate in overload resolution unless zoned_time is constructible from the return type of zoned_traits<TimeZonePtr>::locate_zone(name), zoned_time, and choose.

Effects: Equivalent to construction with {locate_zone(name), y, c}.

Note: The choose parameter is allowed here, but has no impact.
zoned_time<Duration, TimeZonePtr>& zoned_time<Duration, TimeZonePtr>::operator=(const sys_time<Duration>& st);
Effects: After assignment get_sys_time() == st. This assignment has no effect on the return value of get_time_zone().

Returns: *this.
zoned_time<Duration, TimeZonePtr>& zoned_time<Duration, TimeZonePtr>::operator=(const local_time<Duration>& lt);
Effects: After assignment get_local_time() == lt. This assignment has no effect on the return value of get_time_zone().

Returns: *this.
zoned_time<Duration, TimeZonePtr>::operator sys_time<duration>() const;
Returns: get_sys_time().
explicit zoned_time<Duration, TimeZonePtr>::operator local_time<duration>() const;
Returns: get_local_time().
TimeZonePtr zoned_time<Duration, TimeZonePtr>::get_time_zone() const;
Returns: zone_.
local_time<typename zoned_time<Duration, TimeZonePtr>::duration> zoned_time<Duration, TimeZonePtr>::get_local_time() const;
Returns: zone_->to_local(tp_).
sys_time<typename zoned_time<Duration, TimeZonePtr>::duration> zoned_time<Duration, TimeZonePtr>::get_sys_time() const;
Returns: tp_.
sys_info zoned_time<Duration, TimeZonePtr>::get_info() const;
Returns: zone_->get_info(tp_).
template <class Duration1, class Duration2, class TimeZonePtr>
bool
operator==(const zoned_time<Duration1, TimeZonePtr>& x,
           const zoned_time<Duration2, TimeZonePtr>& y);
Returns: x.zone_ == y.zone_ && x.tp_ == y.tp_.
template <class Duration1, class Duration2, class TimeZonePtr>
bool
operator!=(const zoned_time<Duration1, TimeZonePtr>& x,
           const zoned_time<Duration2, TimeZonePtr>& y);
Returns: !(x == y).
template <class charT, class traits, class Duration, class TimeZonePtr>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os,
           const zoned_time<Duration, TimeZonePtr>& t)
Effects: Streams t to os using the format "%F %T %Z" and the value returned from t.get_local_time().

Returns: os.
template <class charT, class traits, class Duration, class TimeZonePtr>
basic_ostream<charT, traits>&
to_stream(basic_ostream<charT, traits>& os, const charT* fmt,
          const zoned_time<Duration, TimeZonePtr>& tp);
Effects: First obtains a sys_info via tp.get_info() which for exposition purposes will be referred to as info. Then calls to_stream(os, fmt, tp.get_local_time(), &info.abbrev, &info.offset).

Returns: os.

Back to TOC

Add a new section 23.17.12.7 leap [time.timezone.leap]:

23.17.12.7 class `leap` [time.timezone.leap]

class leap
{
    sys_seconds date_;  // exposition only

public:
    leap(const leap&)            = default;
    leap& operator=(const leap&) = default;
    
    // Undocumented constructors

    sys_seconds date() const;
};

bool operator==(const leap& x, const leap& y);
bool operator!=(const leap& x, const leap& y);
bool operator< (const leap& x, const leap& y);
bool operator> (const leap& x, const leap& y);
bool operator<=(const leap& x, const leap& y);
bool operator>=(const leap& x, const leap& y);

template <class Duration> bool operator==(const const leap&         x, const sys_time<Duration>& y);
template <class Duration> bool operator==(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator!=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator!=(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator< (const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator< (const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator> (const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator> (const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator<=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator<=(const sys_time<Duration>& x, const leap&               y);
template <class Duration> bool operator>=(const leap&               x, const sys_time<Duration>& y);
template <class Duration> bool operator>=(const sys_time<Duration>& x, const leap&               y);

leap is a copyable class that is constructed and stored in the time zone database when initialized. You can explicitly convert it to a sys_seconds with the member function date() and that will be the date of the leap second insertion. leap is equality and less-than comparable, both with itself, and with sys_time<Duration>.

[Example:

Here is the date of all of the leap second insertions at the time of this writing:

    for (auto& l : get_tzdb().leaps)
        cout << l.date() << '\n';

which outputs:

    1972-07-01 00:00:00
    1973-01-01 00:00:00
    1974-01-01 00:00:00
    1975-01-01 00:00:00
    1976-01-01 00:00:00
    1977-01-01 00:00:00
    1978-01-01 00:00:00
    1979-01-01 00:00:00
    1980-01-01 00:00:00
    1981-07-01 00:00:00
    1982-07-01 00:00:00
    1983-07-01 00:00:00
    1985-07-01 00:00:00
    1988-01-01 00:00:00
    1990-01-01 00:00:00
    1991-01-01 00:00:00
    1992-07-01 00:00:00
    1993-07-01 00:00:00
    1994-07-01 00:00:00
    1996-01-01 00:00:00
    1997-07-01 00:00:00
    1999-01-01 00:00:00
    2006-01-01 00:00:00
    2009-01-01 00:00:00
    2012-07-01 00:00:00
    2015-07-01 00:00:00
    2017-01-01 00:00:00

— end example]

sys_seconds leap::date() const

Returns: date_.

bool operator==(const leap& x, const leap& y)

Returns: x.date() == y.date().

bool operator!=(const leap& x, const leap& y)

Returns: !(x == y).

bool operator<(const leap& x, const leap& y)

Returns: x.date() < y.date().

bool operator>(const leap& x, const leap& y)

Returns: y < x.

bool operator<=(const leap& x, const leap& y)

Returns: !(y < x).

bool operator>=(const leap& x, const leap& y)

Returns: !(x < y).

template <class Duration> bool operator==(const const leap& x, const sys_time<Duration>& y)

Returns: x.date() == y.

template <class Duration> bool operator==(const sys_time<Duration>& x, const leap& y)

Returns: y == x.

template <class Duration> bool operator!=(const leap& x, const sys_time<Duration>& y)

Returns: !(x == y).

template <class Duration> bool operator!=(const sys_time<Duration>& x, const leap& y)

Returns: !(x == y).

template <class Duration> bool operator< (const leap& x, const sys_time<Duration>& y)

Returns: x.date() < y.

template <class Duration> bool operator< (const sys_time<Duration>& x, const leap& y)

Returns: x < y.date().

template <class Duration> bool operator> (const leap& x, const sys_time<Duration>& y)

Returns: y < x.

template <class Duration> bool operator> (const sys_time<Duration>& x, const leap& y)

Returns: y < x.

template <class Duration> bool operator<=(const leap& x, const sys_time<Duration>& y)

Returns: !(y < x).

template <class Duration> bool operator<=(const sys_time<Duration>& x, const leap& y)

Returns: !(y < x).

template <class Duration> bool operator>=(const leap& x, const sys_time<Duration>& y)

Returns: !(x < y).

template <class Duration> bool operator>=(const sys_time<Duration>& x, const leap& y)

Returns: !(x < y).

Back to TOC

Add a new section 23.17.12.8 link [time.timezone.link]:

23.17.12.8 class link [time.timezone.link]
class link
{
private:
    std::string name_;    // exposition only
    std::string target_;  // exposition only

public:
    link(const link&)            = default;
    link& operator=(const link&) = default;

    // Undocumented constructors

    const string& name()   const;
    const string& target() const;
};

bool operator==(const link& x, const link& y);
bool operator!=(const link& x, const link& y);
bool operator< (const link& x, const link& y);
bool operator> (const link& x, const link& y);
bool operator<=(const link& x, const link& y);
bool operator>=(const link& x, const link& y);
A link is an alternative name for a time_zone. The alternative name is name(). The name of the time_zone for which this is an alternative name is target(). links will be constructed for you when the time zone database is initialized.
const string& link::name() const
Returns: name_.
const string& link::target() const
Returns: target_.
bool operator==(const link& x, const link& y)
Returns: x.name() == y.name().
bool operator!=(const link& x, const link& y)
Returns: !(x == y).
bool operator< (const link& x, const link& y)
Returns: x.name() < y.name().
bool operator> (const link& x, const link& y)
Returns: y < x.
bool operator<=(const link& x, const link& y)
Returns: !(y < x).
bool operator>=(const link& x, const link& y);
Returns: !(x < y).

Back to TOC

Modify the synopsis in section [fs.filesystem.syn] 30.10.6 Header <filesystem> synopsis:

using file_time_type = chrono::time_point<trivial-clock chrono::file_clock>;

Back to TOC

Add to [thread.req.paramname] 33.2.1 Template parameter names:

1 Throughout this Clause, the names of template parameters are used to express type requirements. If a template parameter is named Predicate, operator() applied to the template argument shall return a value that is convertible to bool. A program that instantiates a template with a template parameter named Clock with a type for which chrono::is_clock_v<Clock> is false, is ill-formed.

Back to TOC

Acknowledgements

A database parser is nothing without its database. I would like to thank the founding contributor of the IANA Time Zone Database Arthur David Olson. I would also like to thank the entire group of people who continually maintain it, and especially the IESG-designated TZ Coordinator, Paul Eggert. Without the work of these people, this software would have no data to parse.

I would also like to thank Jiangang Zhuang and Bjarne Stroustrup for invaluable feedback for the timezone portion of this library, which ended up also influencing the date.h library. Thanks also to Jonathan Wakely for agreeing to present this paper in Oulu for me. Thank you Daniel Krügler for the incredibly thorough review. Thank you Tomasz Kamiński for the very helpful changes to the proposed wording.

And I would also especially like to thank the growing list of contributors to this library.

References

N. Dershowitz and E. Reingold, Calendrical Calculations 3rd ed., Cambridge University Press 2008.

`%a`	The locale's abbreviated weekday name. If the value does not contain a valid `weekday`, `ios::failbit` is set.
`%A`	The locale's full weekday name. If the value does not contain a valid `weekday`, `ios::failbit` is set.
`%b`	The locale's abbreviated month name. If the value does not contain a valid `month`, `ios::failbit` is set.
`%B`	The locale's full month name. If the value does not contain a valid `month`, `ios::failbit` is set.
`%c`	The locale's date and time representation. The modified command `%Ec` produces the locale's alternate date and time representation.
`%C`	The year divided by 100 using floored division. If the result is a single decimal digit, it is prefixed with `0`. The modified command `%EC` produces the locale's alternative representation of the century.
`%d`	The day of month as a decimal number. If the result is a single decimal digit, it is prefixed with `0`. The modified command `%Od` produces the locale's alternative representation.
`%D`	Equivalent to `%m/%d/%y`.
`%e`	The day of month as a decimal number. If the result is a single decimal digit, it is prefixed with a space. The modified command `%Oe` produces the locale's alternative representation.
`%F`	Equivalent to `%Y-%m-%d`.
`%g`	The last two decimal digits of the ISO week-based year. If the result is a single digit it is prefixed by `0`.
`%G`	The ISO week-based year as a decimal number. If the result is less than four digits it is left-padded with `0` to four digits.
`%h`	Equivalent to `%b`.
`%H`	The hour (24-hour clock) as a decimal number. If the result is a single digit, it is prefixed with `0`. The modified command `%OH` produces the locale's alternative representation.
`%I`	The hour (12-hour clock) as a decimal number. If the result is a single digit, it is prefixed with `0`. The modified command `%OI` produces the locale's alternative representation.
`%j`	The day of the year as a decimal number. Jan 1 is `001`. If the result is less than three digits, it is left-padded with `0` to three digits.
`%m`	The month as a decimal number. Jan is `01`. If the result is a single digit, it is prefixed with `0`. The modified command `%Om` produces the locale's alternative representation.
`%M`	The minute as a decimal number. If the result is a single digit, it is prefixed with `0`. The modified command `%OM` produces the locale's alternative representation.
`%n`	A newline character.
`%p`	The locale's equivalent of the AM/PM designations associated with a 12-hour clock.
`%r`	The locale's 12-hour clock time.
`%R`	Equivalent to `%H:%M`.
`%S`	Seconds as a decimal number. If the number of seconds is less than 10, the result is prefixed with `0`. If the precision of the input can not be exactly represented with seconds, then the format is a decimal floating point number with a fixed format and a precision matching that of the precision of the input (or to a microseconds precision if the conversion to floating point decimal seconds can not be made within 18 fractional digits). The character for the decimal point is localized according to the locale. The modified command `%OS` produces the locale's alternative representation.
`%t`	A horizontal-tab character.
`%T`	Equivalent to `%H:%M:%S`.
`%u`	The ISO weekday as a decimal number (1-7), where Monday is 1. The modified command `%Ou` produces the locale's alternative representation.
`%U`	The week number of the year as a decimal number. The first Sunday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. If the result is a single digit, it is prefixed with `0`. The modified command `%OU` produces the locale's alternative representation.
`%V`	The ISO week-based week number as a decimal number. If the result is a single digit, it is prefixed with `0`. The modified command `%OV` produces the locale's alternative representation.
`%w`	The weekday as a decimal number (0-6), where Sunday is 0. The modified command `%Ow` produces the locale's alternative representation.
`%W`	The week number of the year as a decimal number. The first Monday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. If the result is a single digit, it is prefixed with `0`. The modified command `%OW` produces the locale's alternative representation.
`%x`	The locale's date representation. The modified command `%Ex` produces the locale's alternate date representation.
`%X`	The locale's time representation. The modified command `%Ex` produces the locale's alternate time representation.
`%y`	The last two decimal digits of the year. If the result is a single digit it is prefixed by `0`.
`%Y`	The year as a decimal number. If the result is less than four digits it is left-padded with `0` to four digits.
`%z`	The offset from UTC in the ISO 8601 format. For example `-0430` refers to 4 hours 30 minutes behind UTC. If the offset is zero, `+0000` is used. The modified commands `%Ez` and `%Oz` insert a `:` between the hours and minutes: `-04:30`. If the offset information is not available, `ios_base::failbit` will be set.
`%Z`	The time zone abbreviation. If the time zone abbreviation is not available, `ios_base::failbit` will be set.
`%%`	A `%` character.

`%a`	The locale's full or abbreviated case-insensitive weekday name.
`%A`	Equivalent to `%a`.
`%b`	The locale's full or abbreviated case-insensitive month name.
`%B`	Equivalent to `%b`.
`%c`	The locale's date and time representation. The modified command `%Ec` interprets the locale's alternate date and time representation.
`%C`	The century as a decimal number. The modified command `%NC` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands `%EC` and `%OC` interpret the locale's alternative representation of the century.
`%d`	The day of the month as a decimal number. The modified command `%Nd` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command `%EC` interprets the locale's alternative representation of the day of the month.
`%D`	Equivalent to `%m/%d/%y`.
`%e`	Equivalent to `%d` and can be modified like `%d`.
`%F`	Equivalent to `%Y-%m-%d`. If modified with a width, the width is applied to only `%Y`.
`%g`	The last two decimal digits of the ISO week-based year. The modified command `%Ng` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.
`%G`	The ISO week-based year as a decimal number. The modified command `%NG` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 4. Leading zeroes are permitted but not required.
`%h`	Equivalent to `%b`.
`%H`	The hour (24-hour clock) as a decimal number. The modified command `%NH` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command `%OH` interprets the locale's alternative representation.
`%I`	The hour (12-hour clock) as a decimal number. The modified command `%NI` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.
`%j`	The day of the year as a decimal number. Jan 1 is `1`. The modified command `%Nj` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 3. Leading zeroes are permitted but not required.
`%m`	The month as a decimal number. Jan is `1`. The modified command `%Nm` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command `%Om` interprets the locale's alternative representation.
`%M`	The minutes as a decimal number. The modified command `%NM` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified command `%OM` interprets the locale's alternative representation.
`%n`	Matches one white space character. [Note: `%n`, `%t` and a space, can be combined to match a wide range of white-space patterns. For example `"%n "` matches one or more white space characters, and `"%n%t%t"` matches one to three white space characters. — end note]
`%p`	The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command `%I` must precede `%p` in the format string.
`%r`	The locale's 12-hour clock time.
`%R`	Equivalent to `%H:%M`.
`%S`	The seconds as a decimal number. The modified command `%NS` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2 if the input time has a precision convertible to seconds. Otherwise the default width is determined by the decimal precision of the input and the field is interpreted as a long double in a fixed format. If encountered, the locale determines the decimal point character. Leading zeroes are permitted but not required. The modified command `%OS` interprets the locale's alternative representation.
`%t`	Matches zero or one white space characters.
`%T`	Equivalent to `%H:%M:%S`.
`%u`	The ISO weekday as a decimal number (1-7), where Monday is 1. The modified command `%Nu` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 1. Leading zeroes are permitted but not required. The modified command `%Ou` interprets the locale's alternative representation.
`%U`	The week number of the year as a decimal number. The first Sunday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. The modified command `%NU` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.
`%V`	The ISO week-based week number as a decimal number. The modified command `%NV` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.
`%w`	The weekday as a decimal number (0-6), where Sunday is 0. The modified command `%Nw` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 1. Leading zeroes are permitted but not required. The modified command `%Ow` interprets the locale's alternative representation.
`%W`	The week number of the year as a decimal number. The first Monday of the year is the first day of week `01`. Days of the same year prior to that are in week `00`. The modified command `%NW` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required.
`%x`	The locale's date representation. The modified command `%Ex` produces the locale's alternate date representation.
`%X`	The locale's time representation. The modified command `%Ex` produces the locale's alternate time representation.
`%y`	The last two decimal digits of the year. If the century is not otherwise specified (e.g. with `%C`), values in the range [69 - 99] are presumed to refer to the years [1969 - 1999], and values in the range [00 - 68] are presumed to refer to the years [2000 - 2068]. The modified command `%Ny` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands `%Ey` and `%Oy` interpret the locale's alternative representation.
`%Y`	The year as a decimal number. The modified command `%NY` where `N` is a positive decimal integer specifies the maximum number of characters to read. If not specified, the default is 4. Leading zeroes are permitted but not required. The modified command `%EY` interprets the locale's alternative representation.
`%z`	The offset from UTC in the ISO 8601 format. For example `-0430` refers to 4 hours 30 minutes behind UTC. The modified commands `%Ez` and `%Ez` parse a `:` between the hours and minutes and leading zeroes on the hour field are optional: `-4:30`.
`%Z`	The time zone abbreviation or name. A single word is parsed. This word can only contain characters from the basic source character set ([lex.charset] in the C++ standard) that are alphanumeric, or one of `'_'`, `'/'`, `'-'` or `'+'`.
`%%`	A `%` character is extracted.

Extending <chrono> to Calendars and Time Zones

Contents

Revision History

Changes since R3

Changes since R2

Changes since R1

Changes since R0

Introduction

Description

Proposed Wording

Table of Contents for Proposed Wording

23.17.4.4 is_clock [time.traits.is_clock]

23.17.5.10 duration stream insertion [time.duration.io]

23.17.7.2 Class utc_clock [time.clock.utc]

23.17.7.3 Class tai_clock [time.clock.tai]

23.17.7.4 Class gps_clock [time.clock.gps]

23.17.7.5 Class file_clock [time.clock.file]

23.17.7.8 local_time [time.clock.local_time]

23.17.8 Formatting [time.format]

23.17.9 Parsing [time.parse]

23.17.10 The civil calendar [time.calendar]

23.17.10.1 Class last_spec [time.calendar.last]

23.17.10.2 Class day [time.calendar.day]

23.17.10.3 Class month [time.calendar.month]

23.17.10.4 Class year [time.calendar.year]

23.17.10.5 Class weekday [time.calendar.weekday]

23.17.10.6 Class weekday_indexed [time.calendar.weekday_indexed]

23.17.10.7 Class weekday_last [time.calendar.weekday_last]

23.17.10.8 Class month_day [time.calendar.month_day]

23.17.10.9 Class month_day_last [time.calendar.month_day_last]

23.17.10.10 Class month_weekday [time.calendar.month_weekday]

23.17.10.11 Class month_weekday_last [time.calendar.month_weekday_last]

23.17.10.12 Class year_month [time.calendar.year_month]

23.17.10.13 Class year_month_day [time.calendar.year_month_day]

23.17.10.14 Class year_month_day_last [time.calendar.year_month_day_last]

23.17.10.15 Class year_month_weekday [time.calendar.year_month_weekday]

23.17.10.16 Class year_month_weekday_last [time.calendar.year_month_weekday_last]

23.17.10.17 civil calendar conventional syntax operators [time.calendar.operators]

23.17.11 Class time_of_day [time.time_of_day]

23.17.12 Time Zones [time.timezone]

23.17.12.1 The time zone database [time.timezone.database]

23.17.12.1.1 Remote time zone database support [time.timezone.database.remote]

23.17.12.2 Exception classes [time.timezone.exception]

23.17.12.3 Information classes [time.timezone.info]

23.17.12.4 Class time_zone [time.timezone.time_zone]

23.17.12.5 Class zoned_traits [time.timezone.zoned_traits]

23.17.12.6 Class zoned_time [time.timezone.zoned_time]

23.17.12.7 class leap [time.timezone.leap]

23.17.12.8 class link [time.timezone.link]

Acknowledgements

References

Extending `<chrono>` to Calendars and Time Zones

23.17.4.4 `is_clock` [time.traits.is_clock]

23.17.10.1 Class `last_spec` [time.calendar.last]

23.17.10.2 Class `day` [time.calendar.day]

23.17.10.3 Class `month` [time.calendar.month]

23.17.10.4 Class `year` [time.calendar.year]

23.17.10.5 Class `weekday` [time.calendar.weekday]

23.17.10.6 Class `weekday_indexed` [time.calendar.weekday_indexed]

23.17.10.7 Class `weekday_last` [time.calendar.weekday_last]

23.17.10.8 Class `month_day` [time.calendar.month_day]

23.17.10.9 Class `month_day_last` [time.calendar.month_day_last]

23.17.10.10 Class `month_weekday` [time.calendar.month_weekday]

23.17.10.11 Class `month_weekday_last` [time.calendar.month_weekday_last]

23.17.10.12 Class `year_month` [time.calendar.year_month]

23.17.10.13 Class `year_month_day` [time.calendar.year_month_day]

23.17.10.14 Class `year_month_day_last` [time.calendar.year_month_day_last]

23.17.10.15 Class `year_month_weekday` [time.calendar.year_month_weekday]

23.17.10.16 Class `year_month_weekday_last` [time.calendar.year_month_weekday_last]

23.17.11 Class `time_of_day` [time.time_of_day]

23.17.12.4 Class `time_zone` [time.timezone.time_zone]

23.17.12.5 Class `zoned_traits` [time.timezone.zoned_traits]

23.17.12.6 Class `zoned_time` [time.timezone.zoned_time]

23.17.12.7 class `leap` [time.timezone.leap]

23.17.12.8 class `link` [time.timezone.link]