New submission from Paul Ganssle <p.gans...@gmail.com>:
While developing a shim for deprecating pytz, I discovered this issue with the
Pacific/Rarotonga zone:
>>> from datetime import datetime, timedelta
>>> from backports.zoneinfo import ZoneInfo
>>> datetime(1991, 2, 1, tzinfo=ZoneInfo("Pacific/Rarotonga")).dst() /
timedelta(hours=1)
1.0
This reports that the DST offset is 1 hour, but in fact it should be 30
minutes, because from 1978 to 1991, Pacific/Rarotonga alternated between -0930
and -10:
$ zdump -V -c 1990,1993 'Pacific/Rarotonga'
Pacific/Rarotonga Sun Mar 4 09:29:59 1990 UT = Sat Mar 3 23:59:59 1990 -0930
isdst=1 gmtoff=-34200
Pacific/Rarotonga Sun Mar 4 09:30:00 1990 UT = Sat Mar 3 23:30:00 1990 -10
isdst=0 gmtoff=-36000
Pacific/Rarotonga Sun Oct 28 09:59:59 1990 UT = Sat Oct 27 23:59:59 1990 -10
isdst=0 gmtoff=-36000
Pacific/Rarotonga Sun Oct 28 10:00:00 1990 UT = Sun Oct 28 00:30:00 1990 -0930
isdst=1 gmtoff=-34200
Pacific/Rarotonga Sun Mar 3 09:29:59 1991 UT = Sat Mar 2 23:59:59 1991 -0930
isdst=1 gmtoff=-34200
Pacific/Rarotonga Sun Mar 3 09:30:00 1991 UT = Sat Mar 2 23:30:00 1991 -10
isdst=0 gmtoff=-36000
I believe that the error comes from the fact that before 1978, they were on
-1030 time, then they transitioned to -0930, then started alternating between
-0930 and -10:
$ zdump -V -c 1977,1980 'Pacific/Rarotonga'
Pacific/Rarotonga Sun Nov 12 10:29:59 1978 UT = Sat Nov 11 23:59:59 1978 -1030
isdst=0 gmtoff=-37800
Pacific/Rarotonga Sun Nov 12 10:30:00 1978 UT = Sun Nov 12 01:00:00 1978 -0930
isdst=1 gmtoff=-34200
Pacific/Rarotonga Sun Mar 4 09:29:59 1979 UT = Sat Mar 3 23:59:59 1979 -0930
isdst=1 gmtoff=-34200
Pacific/Rarotonga Sun Mar 4 09:30:00 1979 UT = Sat Mar 3 23:30:00 1979 -10
isdst=0 gmtoff=-36000
Pacific/Rarotonga Sun Oct 28 09:59:59 1979 UT = Sat Oct 27 23:59:59 1979 -10
isdst=0 gmtoff=-36000
Pacific/Rarotonga Sun Oct 28 10:00:00 1979 UT = Sun Oct 28 00:30:00 1979 -0930
isdst=1 gmtoff=-34200
This is not amazingly important, but it would be a good idea to make the result
correct.
Right now I think the heuristic looks for the first example of an STD → DST
transition and decides that's the best option. It might be a good idea to
change the heuristic to look at *all* examples of such transitions and choose
the plurality value, and if there's no plurality value and one of the values is
1H, choose that one.
----------
assignee: p-ganssle
components: Library (Lib)
messages: 371136
nosy: p-ganssle
priority: normal
severity: normal
stage: needs patch
status: open
title: zoneinfo gives incorrect dst() in Pacific/Rarotonga between 1978 and 1991
type: behavior
versions: Python 3.10, Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40930>
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