[Datetime-SIG] IANA TZ database statistics

[Tim Peters]

[Tim]
>> So it seems a sufficient condition is that there's at most one UTC
>> offset change in the last 24 hours.

[Alex]
> Yes.  That's the condition I've been talking for months about.

?


> If you have a chance, please take a look at
>
> https://github.com/abalkin/cpython/commit/d146830e70a1fda22380c5ba0d9592c16acd23de
>
> It fails on Europe/Tallinn which seems to have transitions separated by 22
> hours with the *same* utcoffset.
>
> I don't understand why zic would ever produce something like this.

Well, the Tallinn source rules I see include:

2:00 EU EE%sT 1999 Nov  1
2:00 - EET 2002 Feb 21

That is, they decided to stop messing with DST at all effective the
start of November, 1999.  But until then, they were following "EU"
daylight rules.  Which ends DST on the last Sunday of October, which
in 1999 happened to be Oct 31.

So the first switch to EET local Sunday morning was due to EU daylight
time ending, and then the second "switch" to EET at local midnight was
due to Tallinn opting out of DST rules altogether.  Which didn't
change the zone name, DST status, or UTC offset.

The output of zic doesn't appear nearly well defined enough to say
whether that's "a bug" or "a feature", though :-(