okay then
i think the best way to do that is to retrieve from the user his GMT (btw, you can do it from the ip.. search for ip to country script or something like it). the user will fill his GMT to your db and you will calculate the time and zone :}
i think that date difference can do that but i'm not sure. anyway this isn't that hard script and with a little effort you can do this by your own. good luck!
tell us what you thought of.. or if you need any help.