gh-148658: Fix DST timezone name for negative timestamps on Windows

[jiekangtian]

Issue: #148658

Commit f5685a2 added _PyTime_windows_filetime() to support negative timestamps on Windows, but hardcoded tm_isdst = -1 (unknown) for local time. This caused strftime('%Z') to return the wrong timezone name.

For timezones that don't observe DST (checked via DaylightDate.wMonth == 0), set tm_isdst = 0 instead of -1 to ensure strftime('%Z') returns the standard time name.

• Issue: datetime.astimezone() returns DST name for negative timestamp on Windows #148658

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[pganssle]

Thanks for doing this. Can you either add a test or find a test that is currently disabled on Windows because it is failing because of this and enable it?

[jiekangtian]

Thanks for doing this. Can you either add a test or find a test that is currently disabled on Windows because it is failing because of this and enable it?

Thanks for the reminder, Added the test. Verified it fails without the fix and passes with it.

[ynkdir]

DST name is still used for timezone which uses DST.

PS C:\Users\yukih\Downloads\cpython> tzutil.exe /s "Eastern Standard Time"   # set US timezone
PS C:\Users\yukih\Downloads\cpython> .\PCbuild\amd64\python.exe
Python 3.15.0a8+ (heads/fix-datetime-dirty:7398f51b3b3, Apr 17 2026, 21:22:52) [MSC v.1950 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import datetime
>>>
>>> datetime.fromtimestamp(-1).astimezone()
datetime.datetime(1969, 12, 31, 18, 59, 59, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=68400), 'Eastern Daylight Time'))
>>>
>>> datetime.fromtimestamp(0).astimezone()
datetime.datetime(1969, 12, 31, 19, 0, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=68400), 'Eastern Standard Time'))

[jiekangtian]

DST name is still used for timezone which uses DST.

PS C:\Users\yukih\Downloads\cpython> tzutil.exe /s "Eastern Standard Time"   # set US timezone
PS C:\Users\yukih\Downloads\cpython> .\PCbuild\amd64\python.exe
Python 3.15.0a8+ (heads/fix-datetime-dirty:7398f51b3b3, Apr 17 2026, 21:22:52) [MSC v.1950 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import datetime
>>>
>>> datetime.fromtimestamp(-1).astimezone()
datetime.datetime(1969, 12, 31, 18, 59, 59, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=68400), 'Eastern Daylight Time'))
>>>
>>> datetime.fromtimestamp(0).astimezone()
datetime.datetime(1969, 12, 31, 19, 0, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=68400), 'Eastern Standard Time'))

Fixed. Improved handling for timezones that use DST.

[ynkdir]

That works fine for me. Thank you.