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

[Thomas M. Steenholdt]

On 01/04/2023 20.26, Paul Eggert wrote:
> Oh, thanks, I'd quite forgotten about that March 15 letter. It seems 
> definitive.

Actually, reading that letter more closely, there is a subtle but 
perhaps important difference to the current tzdb definition:

///Greenland has for many years changed to Daylight Saving Time the same 
period as the European Union does. Greenland continues to do so. 
However, Greenland will not switch to Daylight Saving Time this year, 
2023, because the standard time for Greenland will change from UTC -3 to 
UTC -2. However, Greenland will change to Daylight Saving Time again in 
2024 and onwards.///

So the previous understanding of the implementation is slightly off. 
Instead of simply changing the timezone along with the change away from 
DST in the fall of 2023 (as in current tzdb), they actually declare 2023 
a year with no DST at all and implement the timezone change in March 
instead. From 2024 onward, DST is back in place as usual. The table on 
page two of the letter supports this, in that it states standard time 
starts on March 25 for 2023, where DST would normally start.

Compared to the current tzdb definition, the clock aligns perfectly, but 
the isdst-flag does not. I believe this would be a more technically 
correct implementation (diff attached):

Zone America/Nuuk       -3:26:56 - LMT     1916 Jul 28 # Godthåb
                         -3:00   -       -03     1980 Apr  6 2:00
                         -3:00   EU      -03/-02 2023 Mar 26 1:00u
                         -2:00   -       -02     2023 Oct 29 1:00u
                         -2:00   EU      -02/-01

Current implementation:

America/Nuuk  Sun Mar 26 00:59:59 2023 UT = Sat Mar 25 21:59:59 2023 -03 
isdst=0 gmtoff=-10800
America/Nuuk  Sun Mar 26 01:00:00 2023 UT = Sat Mar 25 23:00:00 2023 -02 
isdst=1 gmtoff=-7200
America/Nuuk  Sun Oct 29 00:59:59 2023 UT = Sat Oct 28 22:59:59 2023 -02 
isdst=1 gmtoff=-7200
America/Nuuk  Sun Oct 29 01:00:00 2023 UT = Sat Oct 28 23:00:00 2023 -02 
isdst=0 gmtoff=-7200
America/Nuuk  Sun Mar 31 00:59:59 2024 UT = Sat Mar 30 22:59:59 2024 -02 
isdst=0 gmtoff=-7200
America/Nuuk  Sun Mar 31 01:00:00 2024 UT = Sun Mar 31 00:00:00 2024 -01 
isdst=1 gmtoff=-3600
America/Nuuk  Sun Oct 27 00:59:59 2024 UT = Sat Oct 26 23:59:59 2024 -01 
isdst=1 gmtoff=-3600
America/Nuuk  Sun Oct 27 01:00:00 2024 UT = Sat Oct 26 23:00:00 2024 -02 
isdst=0 gmtoff=-7200

Proposed implementation:

America/Nuuk  Sun Mar 26 00:59:59 2023 UT = Sat Mar 25 21:59:59 2023 -03 
isdst=0 gmtoff=-10800
America/Nuuk  Sun Mar 26 01:00:00 2023 UT = Sat Mar 25 23:00:00 2023 -02 
isdst=0 gmtoff=-7200
America/Nuuk  Sun Mar 31 00:59:59 2024 UT = Sat Mar 30 22:59:59 2024 -02 
isdst=0 gmtoff=-7200
America/Nuuk  Sun Mar 31 01:00:00 2024 UT = Sun Mar 31 00:00:00 2024 -01 
isdst=1 gmtoff=-3600
America/Nuuk  Sun Oct 27 00:59:59 2024 UT = Sat Oct 26 23:59:59 2024 -01 
isdst=1 gmtoff=-3600
America/Nuuk  Sun Oct 27 01:00:00 2024 UT = Sat Oct 26 23:00:00 2024 -02 
isdst=0 gmtoff=-7200

What's your take on this?

/Thomas
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