How to change system time zone in Windows programmatically

I’ve been writing software that allows to change the current time zone parameters used in Windows. This kinda stuff:

The only reference to setting a system-wide time zone that I found was the SetTimeZoneInformation API (or its variation SetDynamicTimeZoneInformation.) But I wasn’t really sure how I can use it to change the current time zone.

For instance, it takes TIME_ZONE_INFORMATION struct with all kinds of information about local time offset, daylight saving settings, etc. I don’t understand why do I need to fill all of this info out when all I want to do is, say change the current time zone from "(UTC-08:00) Pacific Time (US & Canada)" to "(UTC-07:00) Mountain Time (US & Canada)".

I needed to look further …

The Answer

So let me try to recap what I ended up doing. Unfortunately my solution was OS specific:

• For Windows Vista & later:
  1. Set SE_TIME_ZONE_NAME privilege.
  2. Call SetDynamicTimeZoneInformation by specifying the details of the time zone to set.
  3. Remove SE_TIME_ZONE_NAME privilege.
• For Windows XP:
  1. Set SE_SYSTEMTIME_NAME privilege.
  2. Call SetTimeZoneInformation with information about the time zone to set.
  3. Reset SE_SYSTEMTIME_NAME privilege back.

Lastly for both OS, broadcast the following message to let running applications know that the time zone has changed:

SendMessageTimeout(HWND_BROADCAST, 
        WM_SETTINGCHANGE, 0, (LPARAM)L"intl",
        SMTO_ABORTIFHUNG,
        2 * 1000,   //the total wait time can be up to the value of uTimeout multiplied by the number of top-level windows.
        NULL);

To get the information on the current time zone use GetDynamicTimeZoneInformation or GetTimeZoneInformation for WinXP.

Additionally you may want to retrieve the list of all available timezones that you may use to choose the info for the new (default) timezone. You can get it from enumerating/parsing the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones

Check here and here for the information on its structure.