WG14 N984

Defect Report #2dd

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Submitter:Randy Meyers (US)
Submission Date: 2002-09-26
Source: Emmanuel Ruffin (ruffin@besancon.sema.slb.com) via ANSI
Reference Document:
Version: 1.0
Subject: struct tm, member tm_isdst, and mktime() in <time.h>


If it is not known whether daylight savings time is in effect (tm_isdst set to -1), some times expressed in struct tm become ambiguous. There is no specification as to what mktime() should do for such cases.


  1. Normally when calling mktime(), the user will set tm_isdst to -1 to request that mktime() determine the true value (See footnote 267). Usually, mktime() can determine whether daylight savings time is in effect based on the time and date information initially stored in the struct tm argument. However, during the Fall change over, there is one hour that exists both in daylight savings time and standard time. Example: In France, we will change time on October the 27th at 3am. That means that at 3am it will be 2am again. If asked to convert October 27 at 2.30am when tm_isdst is -1, what value should mktime() store in tm_isdst and what is the return value?
  2. For the same example as point 1, what should we return in case tm_isdst is set to 0 or 1 ?
  3. In a general case, what should we do in case tm_isdst is different from -1 ?
  4. When calling mktime function, is it true that this function should modify the tm structure to put in it the GM time instead the local time given as an entry ?

Suggested Committee Response

Subclause 7.23.1 Paragraph 1 of the C Standard says, "The local time zone and Daylight Saving Time are implementation-defined." That means that the standard does not specify the behavior and that the implementation is free to make choices that it must document. Although the C Standard imposes no particular definition on daylight savings time, other standards or local custom may.

  1. It is implementation defined. For example, an implementation might assume that daylight savings time is not in effect and set tm_isdst to 0 and return the time_t value corresponding to 2:30 AM Standard Time.
  2. It is implementation defined. However, assuming that an implementation chose a conventional definition of daylight savings time, these times are unambiguous since the user specified whether daylight savings time was in effect, and the time_t return value would be different for 2:30 daylight savings time versus 2:30 standard time. Note that it would be reasonable for mktime() change tm_hour and tm_isdst on output. For example, tm_hour=2 and tm_isdst=1 on input might change to tm_hour=1 and tm_dst=0 on output.
  3. It is implementation defined. One possibility would be to consider any two struct tm values as being exactly one hour apart if all members have the same value except that one struct tm value has tm_isdst=1 and the other has tm_isdst=0 (regardless of the date stored in the struct tm values).
  4. No. A struct tm represents a local time in the local time zone for mktime(). See Paragraph 2.

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