Late reply, but:

This is absolutely impossible without knowing local rules. Because there can be different rules within a single timezone. Example: while Arizona in its entirety belong to the Mountain time zone, most (but not all) of it does NOT observe Daylight Savings Time. Other states in this same timezone do. There are a number of other similar situations in the world.

Your best bet is to query some service for example geonames. Then you can have accurate data.