Properly handle systems with non-UTC timezones

The documentation for `datetime.utcnow()` states:

> Because naive `datetime` objects are treated by many `datetime`
> methods as local times, it is preferred to use aware datetimes to
> represent times in UTC. As such, the recommended way to create an
> object representing the current time in UTC is by calling
> `datetime.now(timezone.utc)`.

Accordingly, this updates the code to use timezone-aware `datetime`
objects.