Issue 0136: Does mktime yield a -1 in the spring-forward gap when tm_isdst is -1?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Paul Eggert, Project Editor (P.J. Plauger)
Date: 1994-03-31
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_136.html

Suppose I run the following program in a US environment, where the clocks will jump forward from 01:59:59 to 03:00:00 on April 3, 1994. This program attempts to invoke mktime on a struct tm that represents 02:30:00 on that date. Does the C Standard let mktime return -1 in this case?

#include <stdio.h>
 #include <time.h>
 int main()
        {
        struct tm t;
        time_t r;

        /* 1994-04-03 02:30:00 */
        t.tm_year = 1994 - 1900; t.tm_mon = 3; t.tm_mday = 3;
        t.tm_hour = 2; t.tm_min = 30; t.tm_sec = 0;

        t.tm_isdst = -1; /* i.e. unknown */

        r = mktime(&t);
        if (r == -1)
                printf("mktime failed\n");
        else
                printf("%s", ctime(&r));
        return 0;
        }

The ANSI C Rationale (corresponding to subclause 7.12.2.3) clearly lets mktime yield -1 in the “fall-backward fold” that will occur when the clock is turned back from 01:59:59 to 01:00:00 on October 30, 1994. The question is whether mktime is also allowed to yield -1 in the “spring-forward gap” when the clock is advanced from 01:59:59 to 03:00:00.

This question arose when Arthur David Olson's popular “tz” time zone software was tested using NIST-PCTS:151-2, Version 1.4, (1993-12-03) a test suite put out by the National Institute of Standards and Technology that attempts to test C and Posix conformance. The PCTS package insists that in the above case, mktime must yield a time_t corresponding to either 01:30:00 or 03:30:00; i.e. PCTS rejects Olson's mktime, which yields -1.

This test case differs in an important way from the common practical use of mktime to “add 1” to the output of localtime or gmtime, since those functions normally set tm_isdst to a nonnegative value, whereas tm_isdst is -1 in the case under question.

I suggest that the Committee issue a clarification which makes it clear that mktime can yield -1 in the spring-forward gap when tm_isdst is -1.

Comment from WG14 on 1997-09-23:

Response

The Standard does not specify the behavior precisely enough to preclude mktime from returning a value of (time_t)-1 and leaving the tm_isdst member set to -1 in such situations.