For display purposes, it is often useful to have a GMT offset for each timezone. Since the offset can change when timezones
are redefined, this will have to use a reference date to perform the calculation.

Two methods should be added. One should return the offset in seconds, the other should return a string (GMT+1, GMT-2,
GMT+1:30, etc). Both should take an optional reference date (UTC) as a parameter and use the current date if unspecified.
To determine the offset, period_for_utc(reference).std_offset can be used.