In Professional Edition (Pocket PC) devices the Clock control panel allows you to set a home timezone and a visiting zone and lets you toggle between them. If you need to retrieve the current settings they are stored in the registry in a key called HKEY_LOCAL_MACHINE\Software\Microsoft\Clock in a binary value called "AppInfo". Here is breakdown of that data:-
00,00,00,00,31,00,00,00,55,00,00,00,00,00,00,00,01,00,00,00,80,01,00,00,be,00,00,00,00,00,00,00,00,00,00,00
The first 4 bytes contains an integer which is 1 if in the home zone, and 0 if visiting.
The next 4 bytes always seems to be 0x31
The next 4 bytes are the index into the timezone collection of the home zone (In this example 85 GMT)
The next 4 bytes are unused
The next 4 bytes indicate whether to use DST (In this example 1 true)
The next 4 bytes are always 0x180
The next 4 bytes are the index into the timezone collection of the visiting zone (in this example 190 - New Delhi)
The next 4 bytes are unused
The next 4 bytes indicate whether to use DST in visiting zone (In this example 0 false)
I suspect that the unused values may be connected to the old Cities feature which was originally in the control panel but removed with PPC2003. You may be wondering how to get the timezone information at the specified indexes - in Windows Mobile they are not all stored in the registry as on other Windows CE flavours. They are accessible through POOM - IPOutlookApp.GetTimeZoneFromIndex, or in managed code in Mobile In The Hand.