[tz] Buggy %z behaviour in strftime

Almaz Mingaleev mingaleev at google.com
Thu Jul 21 11:25:55 UTC 2022

Just to make sure I understand it correctly: does it mean that %z will be
replaced by offset used by a time zone at a time specified by tm struct and
TZ env variable so developer won't be able to get DST offset at a time
when standard time is in effect as specified methods override tm_isdst
Or setting the tm_isdst field to the opposite value _after_ any of these
should also change the way strftime behaves?

My understanding of the proposed fix is that users won't be able to get DST
time when standard is used and vice-versa. That removes ambiguity around
certain dates at some time zones, I like that.

On Mon, 18 Jul 2022 at 18:19, Paul Eggert <eggert at cs.ucla.edu> wrote:

> On 7/18/22 09:03, Paul Eggert wrote:
> >
> > I suppose someone should file a defect report with the C standardization
> > committee.
> I found what I hope is the correct email address for that (it's not
> well-advertised) and submitted the following bug report:
> ----
> Subject: strftime %z and %Z depend on more than just tm_isdst
> A recent discussion in the Time Zone Database mailing list
> <https://mm.icann.org/pipermail/tz/2022-July/031674.html> has prompted
> me to file this bug report against the C standard.
> Draft N2912, section "The strftime function", pages 365-6,
> paragraph 3 says that the %z and %Z conversion specifiers examine only
> the tm_isdst members of the passed-in structure. However, this is not
> how many implementations actually behave.
> Some implementations (e.g., AIX, Solaris) infer %z and %Z output from
> tm_year, tm_mon, tm_mday, tm_hour, tm_min, tm_sec, and tm_isdst, and
> therefore sometimes guess %z or %Z output incorrectly when the clock
> moves back from one standard time to another (e.g., Iran at the end of
> the year 1978). These implementations typically also depend on the
> current setting of the TZ environment variable.
> Other implementations (e.g., FreeBSD, GNU/Linux) get the UT offset and
> abbreviation from struct tm members tm_gmtoff and tm_zone that are not
> specified by the C standard. These implementations do not have to guess
> %z and %Z output; however, they require that the nonstandard members be
> filled in correctly by localtime or equivalent.
> Implementations that conform to the standard's current wording cannot
> handle anything more complicated than a time zone with just one standard
> time offset and abbreviation, which means they cannot handle timestamps
> for locations like Iran, Portugal, etc., that have changed their
> standard-time offset. The C standard should not require implementations
> to be so limited that they cannot handle common timekeeping situations.
> A simple fix for this issue is to require applications to initialize a
> struct tm as if by localtime or equivalent, before passing the struct tm
> to strftime. This is what applications need to do anyway, if they want
> to be portable to real-world systems such as AIX, FreeBSD, GNU/Linux,
> Solaris, etc.
> Proposed fix:
> 1. Remove the two instances of "[tm_isdst]" in the %z and %Z entries in
> Draft N2912, section "The strftime function", pages 365-6,
> paragraph 3.
> 2. Add the following text to that paragraph:
> If the %z and %Z conversion specifiers are used, the broken-down time
> structure pointed to by timeptr shall contain values generated by a
> successful previous call to gmtime, gmtime_r, gmtime_s, localtime,
> localtime_r, localtime_s, or mktime.
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