...
Times are not converted to GMT as soon as you enable the time zone feature in setup. Immediately after you turn on the time zone view feature, the system will attempt to populate certain specific date/time related fields in GMT in the database for new records added. Existing service call, time stamp, and appointment data must be converted after you populate certain tables with time zone values. Existing data must be converted before you add new service calls or appointments.
Service call, time stamp, and appointment get the time zone from location.
...