Derick Rethans has posted a method he's come up with to try to figure out the user's timezone based on the IP address they're using:

Through Planet PHP I found an article on Pre-populating forms with the timezone. I'd normally add a comment instead, but the comment would almost be larger then the original post, so I am instead writing up an entry myself. The post describes several ways to obtain the user's timezone and use that to pre-fill a form. None of them are working properly though.

In Derick's method combines a bit of Javascript with some PHP to correctly find the timezone.