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
Date:
Subject: struct tm, member tm_isdst, and mktime() in <time.h>
Summary
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.
Questions
- Normally when calling mktime(), the user will set tm_isdst to -1
to request that mktime() determine the true value (See 7.23.2.3
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?
-
For the same example as point 1, what should we return in case
tm_isdst is set to 0 or 1 ?
-
In a general case, what should we do in case tm_isdst is different
from -1 ?
-
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.
-
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.
-
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.
-
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).
-
No. A struct tm represents a local time in the local time zone for
mktime(). See 7.23.2.3 Paragraph 2.
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