I am working with datetime objects in python. I have a function that takes a time and finds the different between that time and now.
now = datetime.now()
diff = now - past_time
datetime doesn't do any cross time zone calculations, because it's a complex and involved subject.
I suggest converting dates to UTC universally and performing maths on those.
I recently completed a project using timezones in a large python/Django project and after investigation went with converting everything internally to UTC and converting only on display to the user.
You should look into pytz to do the conversions to/from UTC, and store Olson codes for the timezones you want in your app - perhaps associated with each user, or appropriate to your program.