Defect Report #136
Submission Date: 31 Mar 94
Submittor: Project Editor (P.J. Plauger)
Source: Paul Eggert
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?
struct tm t;
/* 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)
The ANSI C Rationale (corresponding to subclause 18.104.22.168) 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
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.
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.
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