[Bug time/34480] mktime fails to determine DST for America/Vancouver from tzdata 2026b

https://sourceware.org/bugzilla/show_bug.cgi?id=3D34480

--- Comment #5 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Aurelien Jarno from comment #4)
>> If the timezone is one that includes Daylight Saving Time (DST) adjustme=
nts
>
> I can try to implement that if it makes sense to do so.

If understand the challenge correctly, POSIX assumes a world where a region=
 has
only one TZ rule that applies to all times. So it makes sense to have this
condition. Things are less clear what this means in the tzdata world, where=
 a
region can have DST in one year, but not in the year when you're trying to
convert a time.

With a single rule, POSIX can also easily determine what the DST offset is
without determining whether DST is in effect. We can't do that, either.

If we assume that system tzdata is always correct, it's probably best drop =
the
adjustment code altogether from glibc. It won't be possible for advanced us=
ers
to work around incorrect transition dates due to outdated system informatio=
n,
though. It's also not clear how we would even implement that because tzdata
does not use tm_isdst =3D=3D 1 for permanent DST like in America/Vancouver.

Not sure if this makes sense. We should probably discuss it on the mailing
list?

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