How can I change the TZ in Win NT?

Thank you first of all.
I am trying to change the TZ in my compunter using the puntenv("TZ=EST5EDT") and _tzset(), it changes the enviroment variable but still the control panel remains without changing the TZ, and I do not know if I am doing something wrong or this TZ only changes for that session and not for the system.

Thanks again,

See the SetTimeZoneInformation() docs in MSDN.