Slick Time Zone Coversions

Hey, I wanted to see if anybody has a some ideas or code for a good time zone coversion function. Basically, I would like something where the user can input their timezone (or if php doesn't recognize timezones, maybe just the current time for them, and it can caluculate the difference, but timezones would be preferred). They then enter a time (hh:mm am/pm) which is not local to them, and the function will output a time that has been converted to their timezone. Basically i just want users to be able to take a non-local time and get the local time in their timezone with it. This will be for US times only also. Thanks a bunch!