I'm doing some datetime magic and trying to figure out why the hours are slightly off.
3pm Central on June 3rd 2014 as UTC:
>>> chicago = pytz.timezone('US/Central')
>>> chicago.localize(datetime(2014,6,3,15,0,0)).astimezone(pytz.utc)
datetime.datetime(2014, 6, 3, 20, 0, tzinfo=<UTC>)
3pm Central on December 2nd 2014 as UTC:
>>> chicago.localize(datetime(2014,12,2,15,0,0)).astimezone(pytz.utc)
datetime.datetime(2014, 12, 2, 21, 0, tzinfo=<UTC>)
The hour in the second example is 21 as opposed to 20 in the first example. I thought maybe the date isn't normalized, so I tried this:
>>> chicago.normalize(chicago.localize(datetime(2014,12,2,15,0,0))).astimezone(pytz.utc)
datetime.datetime(2014, 12, 2, 21, 0, tzinfo=<UTC>)
It's still 21. What's going on here?
Daylight Savings Time.
In the first case (Jun 3), Chicago is under Central Daylight Savings Time. The offset to UTC is five hours.
>>> fmt = '%Y-%m-%d %H:%M:%S %Z%z'
>>> clt=chicago.localize(datetime(2014,6,3,15,0,0))
>>> clt.strftime(fmt)
'2014-06-03 15:00:00 CDT-0500'
^^^^^^^^
In the second case (Dec 2), Chicago is under Central Standard Time. The offset to UTC is six hours.
>>> clt=chicago.localize(datetime(2014,12,2,15,0,0))
>>> clt.strftime(fmt)
'2014-12-02 15:00:00 CST-0600'
^^^^^^^^
Your call to normalize() doesn't help here, because you're not doing date or time arithmetic on local times that cross DST boundaries.