[New-bugs-announce] [issue40930] zoneinfo gives incorrect dst() in Pacific/Rarotonga between 1978 and 1991

[Paul Ganssle]

New submission from Paul Ganssle <p.ganssle at 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 <report at bugs.python.org>
<https://bugs.python.org/issue40930>
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