[pywin32-bugs] [ pywin32-Bugs-3523104 ] TimeZoneInfo does not convert timezones correctly

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Bugs item #3523104, was opened at 2012-05-02 13:01
Message generated for change (Comment added) made by jaraco
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Category: win32timezone
Group: None
>Status: Closed
Resolution: None
Priority: 5
Private: No
Submitted By: Brian Matthews (bmatthews27)
Assigned to: Jason R. Coombs (jaraco)
Summary: TimeZoneInfo does not convert timezones correctly

Initial Comment:
in the following example two dates are converted from Hawaii time zone to Pacific time zone. In both cases the time difference should be 3 hours.
The first is correct, the second is incorrect

>>> from win32timezone import TimeZoneInfo
>>> from datetime import datetime
>>> tzi = TimeZoneInfo('Hawaiian Standard Time')
>>> tzs = TimeZoneInfo('Pacific Standard Time')
>>> dthaw = datetime(2011, 11, 5, 15, 59, 59, 0, tzinfo=tzi)
>>> print dthaw.timetuple()
time.struct_time(tm_year=2011, tm_mon=11, tm_mday=5, tm_hour=15, tm_min=59, tm_sec=59, tm_wday=5, tm_yday=309, tm_isdst=0)
>>> dtpac = dthaw.astimezone(tzs)
>>> print dtpac.timetuple()
time.struct_time(tm_year=2011, tm_mon=11, tm_mday=5, tm_hour=18, tm_min=59, tm_sec=59, tm_wday=5, tm_yday=309, tm_isdst=1)
>>>
>>> dthaw = datetime(2011, 11, 5, 16, 0, 0, 0, tzinfo=tzi)
>>> print dthaw.timetuple()
time.struct_time(tm_year=2011, tm_mon=11, tm_mday=5, tm_hour=16, tm_min=0, tm_sec=0, tm_wday=5, tm_yday=309, tm_isdst=0)
>>> dtpac = dthaw.astimezone(tzs)
>>> print dtpac.timetuple()
time.struct_time(tm_year=2011, tm_mon=11, tm_mday=5, tm_hour=17, tm_min=0, tm_sec=0, tm_wday=5, tm_yday=309, tm_isdst=1)
>>>



 

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>Comment By: Jason R. Coombs (jaraco)
Date: 2012-05-04 17:37

Message:
Brian, thanks for your help and patience. The problem is fixed in
085048997feb.

You want the utc offset for a utc timestamp in a specified timezone?

>>> timestamp = datetime(2011, 11, 6, 4, 0, tzinfo=TimeZoneInfo.utc())
>>> dt_pac = timestamp.astimezone(TimeZoneInfo('Pacific Standard Time'))
>>> dt_pac.utcoffset().total_seconds() / 3600
-7.0

Do you think win32timezone should have a single function that takes a naive
timestamp in utc and a time zone name and returns the offset? Something
like offset_for_utc_timestamp_in_zone(datetime, zone_name) ?



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Comment By: Brian Matthews (bmatthews27)
Date: 2012-05-04 16:30

Message:
I think the problem is that TimeZoneInof only works with Naive datetime.
That is, if you pass in a datetime in UTC, the utcoffset() returns as if it
were localtime.
EG:

from win32timezone import TimeZoneInfo
from datetime import datetime
import calendar
import time
tzi = TimeZoneInfo('Pacific Standard Time')
tzutc = TimeZoneInfo('Coordinated Universal Time')
tz_time = datetime(2011, 11, 6, 4, 0, 0, tzinfo=tzutc)
tzi.utcoffset(tz_time).total_seconds()/60/60

shows -8:00 when it should be -7:00 because the datetime is UTC, not
localtime.
We have a way around this rght now by doing a 2-pass calculation tto see if
we crossed the pst/dst time.

win32timezone needs a method of showing the utcoffset for a utc timestamp
in the specified timezone

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Comment By: Jason R. Coombs (jaraco)
Date: 2012-05-03 05:27

Message:
I've started looking into this, and I've expanded the tests you provided:
http://a.libpa.st/mP7l7

According to the docs, astimezone should be converting from a very strange
form (the time in UTC with the pacific time zone indicated), and fromutc
should handle that correctly as long as the standard offset isn't changing
on an hour-by-hour basis (which it is not for either of these time zones).

http://docs.python.org/library/datetime.html#datetime.datetime.astimezone
http://docs.python.org/library/datetime.html#datetime.tzinfo.fromutc

I'll continue to investigate, but this is beginning to look like a bug in
the datetime implementation (or at least the docs).

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Comment By: Brian Matthews (bmatthews27)
Date: 2012-05-02 13:11

Message:
This was using the patched code from #3521185

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