think you'll need java script for that- php is server side, and therefore doesnt really have anything to do with the end users computer - it just sends them a load of code. Therefore, if you change your clock where you are, php will still look at the clock on the server that it's running on to get a time stamp.
you would need javascript (which is client side) if you wanted to get information from the end users clock, and i'm not sure how to do that. SOrry!