Hope my recent answer to a similar Question will help you to understand all this:
[Solved] XML Date Time Conversion Issue[
^].
[EDIT]
My other recent answers shows how to list the time zones:
GMT Time using particular country name[
^].
You can generate a list of zones at the server, present it to the user and ask the user to create an account and select one of the time zones. Make it mandatory by server-side validation, don't allow for default zone. In this way, you will be able to do time calculations depending on the account's time zone data.
—SA