[tz] Did Greenland abolish daylight saving from 2024 on?

Guy Harris gharris at sonic.net
Tue Nov 14 00:48:26 UTC 2023

On Nov 13, 2023, at 3:47 PM, Paul Eggert via tz <tz at iana.org> wrote:

> As you say, real software by and large doesn't care about tm_isdst any more. This is why we haven't bothered to issue a new TZDB release yet - nobody cares in the real world. But eventually when the next TZDB release comes out I'd like the tm_isdst flag to reflect better the intended rules by Greenland's government, as  we might as well do things right if it's easy. I expect will care about this except us TZDB nerds.

I see three ways in which tm_isdst can be used:

	1) as an indication of whether something known as "daylight saving time" or "summer time" or something other than "standard time" is in effect;

	2) as an indication of whether the offset-from-UTC is advanced from what it would be in winter;

	3) as an index into the tzname[] array.

Given that, in the real world, clocks do not all follow a simple "standard time" vs "daylight saving time" model, with, for example, the Moroccan adjustments for Ramadan and the Irish "we call what we get when we advance the clocks for summer 'standard time' and we call what we get when we turn back the clocks for winter 'winter time', rather than what most locales that follow that pattern do", I'd say that it'd be unwise to write code that does 1).

2) and 3) could be made to work, perhaps with some trickery to handle cases where, in the period 1970-present, the are more than to abbreviations for tzname[], e.g. with the tzname[] array being set in such a way that it works for the last "struct tm" values supplied by the library, and thus changed multiple times.

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