time

I am writing a program for managing leads and contacts. I would like to add the ability to see what TIME it is where the contact is not there server time. So if you looked at a list of contacts from all over the country you would see different times compared to what time it is where the user is at. What should I be looking at to calculate out the different time zones. Could somebody just point me in the write direction at this time I have no idea on how to proceeded.