When you are operating across multiple time-zones, one way is to record one time-stamp:

• A common standard date along with the flag of the time-zone in which the event occurred, so that local time can be calculated at run-time.
• The local time-stamp with the flag of the time-zone in which the event occurred so that the common reference time (say GMT) can be calculated.

This is not advisable, because it will put run-time load (as time dimension is most used dimension), and this run-time calculation may need to be plugged into various reports and queries that you are generating. Moreover sometimes the rules for calculating the local time given the GMT and vice-versa may not be that straight. Therefore it is advised to have both the time-stamps relating to two different time zone. However this should be done only for key time-zones for which you will need your reporting on. For the other time-zones, which could be used on occasional basis, one can do real time calculations.