In the old days of running Linux, I used to change timezone of a machine by running time —set= at the command line and simply including the timezone code (PST). These days, Debian, Ubuntu, and the other similar derivatives have made life so much easier. If you want to change the timezone, all you need to do is run (as root):
dpkg-reconfigure tzdata
It will pop up a little config screen, asking you where you are and will summarily set your timezone. These is much easier than the old way since you don’t even have to know your own timezone code (though… who wouldn’t?).