This will avoid Daylight Saving Time issues and resolves a Django
warning about using naive datetime objects ("RuntimeWarning:
DateTimeField received a naive datetime").
This adds a dependency on python-tz.
https://docs.djangoproject.com/en/dev/topics/i18n/timezones/
(imported from commit 10e432a67e2bcd014a0cedf4423a162609873825)