You could have the reset script check the timezone of the user before actually performing the reset, and have this script run at least once an hour to make sure you get close to accurate for most people (some timezones only differ by less than an hour though!).

But also, if the user can change the timezone themselves, now you have a condition where the user could keep their quota at bay for a very long time (if not forever) if they keep changing their timezone accordingly.