Locale

Hello folks,
Can anyone tell me how can i get to know a person's time zone from his request.Actually you will suggest through Locale but locale is browser specific.A person can always be in US and change his browser locale to india and i would come to know he is in india and would show him information based on indian standards.
I am supposed to show time to every person as per his time zone.So is there any way that i can actually know where or in which country he is so that i can manipulate GMT time and show him his zone specific time.
Any piece od code for that ?

Hope to get some response in this regard,
Saurabh