Most datetime objects returned from the dateutil parser are naïve, meaning they don't have an explicit tzinfo, which specifies the timezone and UTC offset. In the previous recipe, only one of the examples had a tzinfo, and that's because it's in the standard ISO format for UTC datetime strings. UTC is the coordinated universal time, and is basically the same as GMT. ISO is the International Standards Organization, which among other things, specifies standard datetime formatting.

Python datetime objects can either be naïve or aware. If a datetime object has a tzinfo, then it is aware. Otherwise, the datetime is naïve. To make a naïve datetime object timezone aware, you must give it an explicit tzinfo. However, the Python datetime library only defines an abstract baseclass for tzinfo, and leaves it up to others to actually implement tzinfo creation. This is where the tz module of dateutil comes in—it provides everything you need to look up timezones from your...