From what I've read, doing a local timezone is not a simple issue.
That's why most do {{datetime|timesince}} relative to a timestamp
whenever possible.
Also, one professor has even suggested that the whole world abandon
timezones entirely and everybody just go with UTC, but that's another
story!
Guido covered it in his talk when he suggested adding 'tz.py' to your
program, and then doing the math based on a User object field before
the response is returned.
Anyway, I'm not sure how Gmail does it, but I don't think I was asked
for my timezone... there's lots of discussion on this topic out there
- Good Luck!