How do I get a value of datetime.today() in Python that is “timezone aware”?

I am trying to subtract one date value from the value of datetime.datetime.today() to calculate how long ago something was. But it complains:

TypeError: can't subtract offset-naive and offset-aware datetimes

The return value from datetime.datetime.today() doesn’t seem to be “timezone aware”, while my other date value is. How do I get a return value from datetime.datetime.today() that is timezone aware?

The ideal solution would be for it to automatically know the timezone.

Right now, it’s giving me the time in local time, which happens to be PST, i.e. UTC – 8 hours. Worst case, is there a way I can manually enter a timezone value into the datetime object returned by datetime.datetime.today() and set it to UTC-8?

Answer

In the standard library, there is no cross-platform way to create aware timezones without creating your own timezone class. (Edit: Python 3.9 introduces zoneinfo in the standard library which does provide this functionality.)

On Windows, there’s win32timezone.utcnow(), but that’s part of pywin32. I would rather suggest to use the pytz library, which has a constantly updated database of most timezones.

Working with local timezones can be very tricky (see “Further reading” links below), so you may rather want to use UTC throughout your application, especially for arithmetic operations like calculating the difference between two time points.

You can get the current date/time like so:

import pytz
from datetime import datetime
datetime.utcnow().replace(tzinfo=pytz.utc)

Mind that datetime.today() and datetime.now() return the local time, not the UTC time, so applying .replace(tzinfo=pytz.utc) to them would not be correct.

Another nice way to do it is:

datetime.now(pytz.utc)

which is a bit shorter and does the same.

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Further reading/watching why to prefer UTC in many cases:

• pytz documentation
• What Every Developer Should Know About Time – development hints for many real-life use cases
• The Problem with Time & Timezones – Computerphile – funny, eye-opening explanation about the complexity of working with timezones (video)