Time zones clearly complicate the use of datetime objects. And because there are so many ways of representing time zones - either using names or offsets from coordinated universal time (UTC) - there is no single technique for managing them. This is made even more complicated by the fact that some time zones have changed over the years. For example, the definition of Eastern Time was different before 1884. Back then, the Eastern Time Zone was 4 minutes ahead of where it is now. That is why the object >>> eastern = pytz.timezone('US/Eastern') can produce unexpected results if you use it incorrectly. For example, if try both >>> t1 = time.replace(tzinfo=eastern) >>> t2 = eastern.localize(time) then t2 will be correct but t1 will use the old pre-1884 version of Eastern! If you read the discussion of localized time and date arithmetic, they recommend to avoid this problem by always using UTC time. But this may not always be possible. Why not