The time zone settings in Windows are kept in registry, in two places. One has a list of all available time zones and their settings and the other has the currently set time zone. When the zone changes Windows copies the proper settings from the list to the current settings key.
The list of time zones is kept under
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones
The current zone is set in
So, here are a few ways of doing that.