pytz brings the Olson tz database into Python. This library allows accurate
and cross platform timezone calculations using Python 2.3 or higher. It also
solves the issue of ambiguous times at the end of daylight savings, which you
can read more about in the Python Library Reference (datetime.tzinfo).

Amost all (over 540) of the Olson timezones are supported [*].

Note that if you perform date arithmetic on local times that cross DST
boundaries, the results may be in an incorrect timezone (ie. subtract 1
minute from 2002-10-27 1:00 EST and you get 2002-10-27 0:59 EST instead of
the correct 2002-10-27 1:59 EDT). This cannot be resolved without modifying
the Python datetime implementation. However, these tzinfo classes provide a
normalize() method which allows you to correct these values.