Re: [PATCH] manual: Add more documentation for the tm_isdst member of struct tm

[Florian Weimer <fweimer@redhat.com>]

* Paul Eggert:

> On 2022-10-24 14:13, Florian Weimer wrote:
>> that's rather inconsistent with civilian use.
>
> It depends on which civilians you talk to. Irish legislation says that
> IST (UTC+01) is Irish Standard Time, and that Ireland observes "winter 
> time" (i.e., negative daylight saving offset) during winter. Irish
> winter time is called "Greenwich mean time". See:
>
> https://github.com/eggert/tz/blob/main/europe#L359
> https://www.irishstatutebook.ie/eli/1971/act/17/enacted/en/print

I think in other cases, tz goes with actual practices on the ground, not
what the internationally recognized government says what the time should
be.  Just saying.

> There's something similar in Morocco, though in their case it's
> Ramadan time not winter time.
>
>> why isn't isdst zero during the winter as well?
>
> Because Irish daylight saving time is observed in winter not
> summer.

This seems to be a bit of a jump?  Is it an inference from the fact that
summer time is called “Irish Standard Time”, and that the Act gives
permission to change or abolish winter time, but not standard time?  So
winter time is presented as an exception.  Daylight saving time is also
presented as an exception, therefore winter time is daylight saving
time?

I'm just trying to follow the line of though.

> PS. While we're on the subject of documentation, perhaps we should
> also mention that mktime can use tm_gmtoff to disambiguate 
> otherwise-ambiguous time stamps. This is reliable even for situations
> like Volgograd in 2020, and it conforms to POSIX. A bit tricky to 
> document, though.

Huh.  How can we do this tm_gmtoff thing without breaking C
compatibility?  I'm worried that a conforming application might not
initialize tm_gmtoff (or rather __tm_gmtoff) properly.

Thanks,
Florian