I am try creating a datetime object in python using datetime and pytz, the offset shown is wrong.
import datetime
from pytz import timezone
start = datetime.datetime(2011, 6, 20, 0, 0, 0, 0, timezone('Asia/Kolkata'))
print start
The output shown is
datetime.datetime(2011, 6, 20, 0, 0, tzinfo=<DstTzInfo 'Asia/Kolkata' HMT+5:53:00 STD>)
Note that 'Asia/Kolkata' is IST which is GMT+5:30 and not HMT+5:53. This is a standard linux timezone, why do I get this wrong, and how do I solve it?
See: http://bytes.com/topic/python/answers/676275-pytz-giving-incorrect-offset-timezone
In the comments, someone proposes to use tzinfo.localize() instead of the datetime constructor, which does the trick.
>>> tz = timezone('Asia/Kolkata')
>>> dt = tz.localize(datetime.datetime(2011, 6, 20, 0, 0, 0, 0))
>>> dt
datetime.datetime(2011, 6, 20, 0, 0, tzinfo=<DstTzInfo 'Asia/Kolkata' IST+5:30:00 STD>)
UPDATE: Actually, the official pytz website states that you should always use localize or astimezone instead of passing a timezone object to datetime.datetime.
pytz as a default uses the extremely uncommon timezones with minutes offset and doesn't use them when using the method .localize(). This is weird as hell.
localize. The reason localize can work correctly is because it is passed the datetime so it can use it to determine the correct historical context and make adjustments. The zoneinfo module added in Python 3.9 doesn't need localize, I think because datetime was changed to support it better.This has been fixed in python >=3.9 by the zoneinfo module in the standard library. The solution in >= 3.9 is probably to stop using pytz.
In [1]: import datetime
In [2]: from zoneinfo import ZoneInfo
In [3]: start = datetime.datetime(2011, 6, 20, 0, 0, 0, 0, ZoneInfo('Asia/Kolkata'))
In [4]: print(start)
2011-06-20 00:00:00+05:30
The reason for this extremely confusing behavior is that time zones used to not be standardized at :30 or :00 offsets. Around the turn of the 20th century most of them came into a standard offset. In the example in OP, the timezone switched in 1906. For US/Central, this happened in 1901.
from datetime import datetime, timedelta, date
from pytz import timezone
d = datetime.combine(date.today(), time.min)
for tz in ('Asia/Kolkata', "US/Central"):
while d > datetime(1800, 1, 1):
localized = timezone(tz).localize(d)
if localized.isoformat()[-2:] not in ("00", "30"):
print(tz)
print(localized.isoformat())
print(timezone(tz).localize(d + timedelta(days=1)).isoformat())
break
d -= timedelta(days=1)
That outputs:
Asia/Kolkata
1906-01-01T00:00:00+05:21
1906-01-02T00:00:00+05:30
US/Central
1901-12-13T00:00:00-05:51
1901-12-14T00:00:00-06:00
Pytz seems to just use the oldest offset when it doesn't have date information, even if it was a very long time ago. In some very natural constructions like passing tzinfo to the datetime constructor, the timezone object is not given that data.
datetimeinterface can work properly now, as evidenced by thezoneinfomodule introduced in Python 3.9 - it doesn't require alocalizefunction to work properly.