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

[Paul Eggert <eggert@cs.ucla.edu>]

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

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. 
Also, if tm_isdst were always zero, it'd be harder for mktime to 
disambiguate Irish timestamps.

POSIX requires support for this sort of thing. Even if TZDB's 
Europe/Dublin entry were changed to disagree with Irish legislation, 
glibc would still need to support settings like this:

TZ='IST-1GMT0,M10.5.0,M3.5.0/1'

This POSIX-conforming TZ setting follows current Irish practice and 
specifies negative DST during winter.

Perhaps we could mention this possibility in the manual.

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.