pytz giving incorrect offset and timezone

Simon Percivall percivall at gmail.com
Thu Jul 12 13:43:56 CEST 2007


On Jul 12, 11:47 am, Sanjay <skpate... at gmail.com> wrote:
> Hi All,
>
> Using pytz, I am facing a problem with Asia/Calcutta, described below.
>
> Asia/Calcutta is actually IST, which is GMT + 5:30. But while using
> pytz, it was recognized as HMT (GMT + 5:53). While I digged into the
> oslan database, I see the following:
>
> # Zone NAME GMTOFF RULES FORMAT [UNTIL]
> Zone Asia/Calcutta 5:53:28 - LMT 1880 # Kolkata
>                5:53:20 - HMT 1941 Oct # Howrah Mean Time?
>                6:30 - BURT 1942 May 15 # Burma Time
>                5:30 - IST 1942 Sep
>                5:30 1:00 IST 1945 Oct 15
>                5:30 - IST
>
> Searching in this group, I saw a similar problem posted athttp://groups.google.co.in/group/comp.lang.python/browse_thread/threa...
> without any solutions.
>
> I mailed to Stuart and also posted it at the launchpad of pytz, but
> did not get any response.
>
> Unable to know how to proceed further. Any suggestion will be of vital
> help.
>
> thanks
> Sanjay

I don't use pytz myself that often so I can't be sure, but I don't
think it's a bug in pytz.

The problem seems to be that the timezone has changed for the
location. Now, without a date as reference, pytz can't know what
timezone to use when constructing the tzinfo; you might want a date
from the 1800's.

When you're constructing the datetime with the tzinfo argument, you're
saying: use this timezone as the local timezone. datetime_new (the
constructor in C) never calls the tzinfo to verify that the timezone
is still valid, it just uses it.

On the other hand: When you construct a datetime with datetime.now()
and pass a timezone, datetime_now (again, in C) calls the method
fromutz() on the tzinfo object. Now the pytz tzinfo object has a
reference by which to choose the current timezone for the location,
and that's why it's correct when you use datetime.now() but not for a
manual construction.

A "workaround" (or maybe the proper way to do it) is to construct the
datetime without a tzinfo set, and then use the localize() method on
the tzinfo object, this will give you the correct result.

>>> tz = pytz.timezone("Asia/Calcutta")
>>> mydate = datetime.datetime(2007, 2, 18, 15, 35)
>>> print tz.localize(mydate)
2007-02-18 15:35:00+05:30




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