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

Paul Eggert eggert at cs.ucla.edu
Fri Nov 17 16:35:45 UTC 2023

On 2023-11-17 01:00, Guy Harris wrote:

> So does POSIX's use of "Daylight Savings Time" mean that "daylight saving time" could either be a time when the clock is turned forward from standard time *or* could *be* standard time, with the clocks being turned *backwards* from standard time for the winter?
> Or does it really mean that "daylight saving time" can refer to a period in which the clock is turned *backwards* from standard time?

The latter is true. (I don't fully understand what you mean by the 
former, or why it differs from the latter.)

Say you're in Ireland and set TZ='IST-1GMT0,M10.5.0,M3.5.0/1'. POSIX 
says that tm_isdst must be positive when clocks are turned backwards 
from standard time. This longstanding POSIX requirement is independent 
of whatever TZDB does.

Here's the zdump -V output for 2023 when TZ has that setting:

$ zdump -V -c 2023,2024 IST-1GMT0,M10.5.0,M3.5.0/1
IST-1GMT0,M10.5.0,M3.5.0/1  Sun Mar 26 00:59:59 2023 UT = Sun Mar 26 
00:59:59 2023 GMT isdst=1 gmtoff=0
IST-1GMT0,M10.5.0,M3.5.0/1  Sun Mar 26 01:00:00 2023 UT = Sun Mar 26 
02:00:00 2023 IST isdst=0 gmtoff=3600
IST-1GMT0,M10.5.0,M3.5.0/1  Sun Oct 29 00:59:59 2023 UT = Sun Oct 29 
01:59:59 2023 IST isdst=0 gmtoff=3600
IST-1GMT0,M10.5.0,M3.5.0/1  Sun Oct 29 01:00:00 2023 UT = Sun Oct 29 
01:00:00 2023 GMT isdst=1 gmtoff=0

> (And if anybody claims that it means that daylight saving time is in effect during the winter, I'm going to {{cn}} the heck out of that claim.)

Although negative DST is most commonly done in winter, it's not the same 
as "winter time". Negative DST can and has been done for other reasons, 
as in Morocco's current practice of negative DST during Ramadan.

Also, negative DST "saves" just as much time as positive DST "saves", as 
the total amount of daylight "saved" is of course zero either way. 
Negative DST merely shifts some daylight from evening to morning, 
whereas positive DST merely does the reverse. They're symmetric and 
there's no technical reason to say that one is DST while the other is not.

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