Modern Python Standard Library Cookbook

Time-zone-aware datetime

Python datetimes are usually naive, which means they don't know which time zone they refer to. This can be a major problem because, given a datetime, it's impossible to know when it actually refers to.

The most common error in working with dates in Python is trying to get the current datetime through datetime.datetime.now(), as all datetime methods work with naive dates, it's impossible to know which time that value represents.