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

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

On 2022-10-26 04:21, Florian Weimer wrote:
> * Paul Eggert:
> 
>> On 2022-10-25 11:07, Florian Weimer wrote:
>>
>>> I think in other cases, tz goes with actual practices on the ground
>>
>> There is no real practice on the ground here, as nobody outside of
>> libc nerds cares (or should care) whether tm_isdst is zero or
>> positive.
> 
> That's not my experience.  We have heard from customers that they use
> non-negative values in their applications.

Oh, by "practice on the ground" I meant ordinary people and mass media 
and the like, not software developers who have to deal with the tm_isdst 
mess.


> Maybe we should just document that applications should set this field to
> -1 when constructing struct tm data?

Yes, if you're deriving struct tm data from the outside world and you 
have no idea what tm_isdst etc. should be, your apps should set tm_isdst 
to -1. However, if the outside world tells you tm_gmtoff (which is 
pretty common these days, e.g., see the "Date:" line in this email), 
then your apps should also set tm_gmtoff to what the outside world tells 
you, before you call mktime.


> In case of Ireland it seems an artificial complication, though.  The
> perception seems to be that IST is summer time, not standard time

My impression is different, in that people in Ireland typically say 
"summer time" or "winter time". They typically do not say "standard 
time" or "daylight saving time". And as it happens, summer time = 
standard time in Ireland.

Part of the confusion here is that the British English phrase "summer 
time" is not the same thing as the American English phrase "daylight 
saving time". The two phrases mean the same thing for UK and US 
timekeeping, but not for Irish timekeeping. Since the Irish typically 
use British English, they refer to their timekeeping correctly.

I think this confusion is partly why POSIX sometimes says "alternative 
time" instead of "daylight saving time" - it's trying to avoid using 
either British or American English and thus avoid the confusion. Though 
I wish POSIX would stick to just one phrase rather than using two, since 
the two phrases in POSIX mean the same thing. I expect the problem here 
is that the C Standard says "daylight saving time" and POSIX copies from 
that when it has to, and uses "alternative time" when it doesn't. Though 
the use of two terms in POSIX simply adds to the confusion....