Running simple program:
```
#include <stdio.h>
#include <time.h>
int main (void) {
_tzset ();
printf ("%s, %s, %d\n", _tzname[0], _tzname[1], _timezone);
return 0;
}
```
With TZ=UTC prints
```
UTC, , 0
```
And with TZ=PST
```
PST, , 0
```
So it seems your guess is correct.
________________________________
From: Pali Rohár <[email protected]>
Sent: Thursday, November 20, 2025 12:53 AM
To: Kirill Makurin <[email protected]>
Cc: [email protected]
<[email protected]>
Subject: Re: [Mingw-w64-public] [PATCH 7/7] crt: Extend t_time test for
_localtime64 and _mktime64
Seems that TZ=UTC is working, at least test is passing for me.
Maybe it parsed "UTC" as a number (like atoi) and it returned that
offset 0, which is UTC?
On Wednesday 19 November 2025 15:42:56 Kirill Makurin wrote:
> Does CRT really support well-known timezones, like TZ=UTC? I have a feeling
> that it accepts value of TZ variable as long as it can parse it according to
> format documented for _tzset[1].
>
> Format for TZ supported by CRT is subset of what POSIX[2] describes. In
> particular, CRT does not support specifying exact time when the switch
> happens, but only offset from UTC. Microsoft docs say that CRT always uses US
> rules for this, which is very unfortunate.
>
> Another thing that I think worth mentioning - it seems that CRT functions
> _ftime and localtime will always try to reparse value of TZ and update
> timezone information (variables _daylight, _dstbias, _timezone and _tzname).
> If two functions (any of _tzset, _ftime or localtime) are called from
> different threads at the same time, and value of TZ was different upon entry,
> this may result in inconsistent result. I don't think there's much we can do
> about.
>
> [1] https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset
> [2] https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html
> ________________________________
> From: Martin Storsjö <[email protected]>
> Sent: Monday, November 17, 2025 11:18 PM
> To: Pali Rohár <[email protected]>
> Cc: [email protected]
> <[email protected]>
> Subject: Re: [Mingw-w64-public] [PATCH 7/7] crt: Extend t_time test for
> _localtime64 and _mktime64
>
> On Sat, 25 Oct 2025, Pali Rohár wrote:
>
> > ---
> > mingw-w64-crt/testcases/t_time.c | 96 ++++++++++++++++++++++++++++++--
> > 1 file changed, 92 insertions(+), 4 deletions(-)
> >
> > diff --git a/mingw-w64-crt/testcases/t_time.c
> > b/mingw-w64-crt/testcases/t_time.c
> > index 50b07d0083a7..75a15d816d31 100644
> > --- a/mingw-w64-crt/testcases/t_time.c
> > +++ b/mingw-w64-crt/testcases/t_time.c
> > @@ -10,14 +10,17 @@ int __cdecl ftime64(struct __timeb64 *tb64);
> >
> > int main()
> > {
> > - time_t t;
> > - __time32_t t32;
> > - __time64_t t64;
> > + time_t t, t_;
> > + __time32_t t32, t32_;
> > + __time64_t t64, t64_;
> > struct timeb tb;
> > struct _timeb tb_;
> > struct __timeb32 tb32;
> > struct __timeb64 tb64;
> > struct tm *htm;
> > + struct tm tm1;
> > + struct tm tm2;
> > + struct tm tm3;
> > int ret1, ret2, ret3;
> > const char *str;
> > const wchar_t *wstr;
> > @@ -95,7 +98,49 @@ int main()
> > assert (htm->tm_yday == 74);
> > assert (htm->tm_isdst == 0);
> >
> > - /* ctime returns time string in local timezone, so set local timezone to
> > UTC to have test timezone independent */
> > + time_t times[] = {
> > + 1700000000 /* Tue Nov 14 22:13:20 UTC 2023 */,
> > + 1600000000 /* Sun Sep 13 12:26:40 UTC 2020 */,
> > + };
> > + for (size_t i = 0; i < sizeof(times)/sizeof(*times); i++) {
> > + t = times[i];
> > + htm = localtime (&t);
> > + tm1 = *htm;
> > + printf ("localtime(%lld): sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + (long long)t, htm->tm_sec, htm->tm_min, htm->tm_hour,
> > htm->tm_mday, htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday,
> > htm->tm_isdst);
> > + t_ = mktime (htm);
> > + printf ("mktime(): %lld sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + (long long)t_, htm->tm_sec, htm->tm_min, htm->tm_hour,
> > htm->tm_mday, htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday,
> > htm->tm_isdst);
> > + assert (t_ == t);
> > + assert (memcmp (htm, &tm1, sizeof(tm1)) == 0);
> > +
> > + t32 = t;
> > + htm = _localtime32 (&t32);
> > + tm2 = *htm;
> > + printf ("_localtime32(%d): sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + t32, htm->tm_sec, htm->tm_min, htm->tm_hour, htm->tm_mday,
> > htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday, htm->tm_isdst);
> > + t32_ = _mktime32 (htm);
> > + printf ("_mktime32(): %d sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + t32_, htm->tm_sec, htm->tm_min, htm->tm_hour, htm->tm_mday,
> > htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday, htm->tm_isdst);
> > + assert (t32_ == t32);
> > + assert (memcmp (htm, &tm2, sizeof(tm2)) == 0);
> > +
> > + t64 = t;
> > + htm = _localtime64 (&t64);
> > + tm3 = *htm;
> > + printf ("_localtime64(%lld): sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + t64, htm->tm_sec, htm->tm_min, htm->tm_hour, htm->tm_mday,
> > htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday, htm->tm_isdst);
> > + t64_ = _mktime64 (htm);
> > + printf ("_mktime64(): %lld sec=%d min=%d hour=%d mday=%d mon=%d
> > year=%d wday=%d yday=%d isdst=%d\n",
> > + t64_, htm->tm_sec, htm->tm_min, htm->tm_hour, htm->tm_mday,
> > htm->tm_mon, htm->tm_year, htm->tm_wday, htm->tm_yday, htm->tm_isdst);
> > + assert (t64_ == t64);
> > + assert (memcmp (htm, &tm3, sizeof(tm3)) == 0);
> > +
> > + assert (memcmp (&tm1, &tm2, sizeof(tm1)) == 0);
> > + assert (memcmp (&tm2, &tm3, sizeof(tm2)) == 0);
> > + }
> > +
> > + /* ctime, localtime and mktime returns time string in local timezone, so
> > set local timezone to UTC to have test timezone independent */
> > putenv ("TZ=UTC");
>
> In addition to setting TZ=UTC here, is it possible to set some other
> well-known timezone that would be available everywhere, which we could use
> for some testcases for cases closer to the DST switchover time? We could
> do that for both mktime and localtime IMO.
>
> In itself, these test additions probably are fine though - the above would
> be nice extras. But given that corner case handling close to DST
> switchovers, I think it would be good to actually check such a case.
>
> So this patchset in itself looks ok, except for the one issue I found in
> 2/7.
>
> // Martin
>
> _______________________________________________
> Mingw-w64-public mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/mingw-w64-public
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