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.

On his blog today, Joakim Nygard has posted about the use (or disuse) of time zones in PHP:

The problem with not handling timezones correctly is not apparent in the usual case of the webserver and the database server being in the same timezone. [...] Often a specific timezone is assumed for displaying date and time. This could very well be incorrect for a large number of visitors. If the database server is in yet another timezone, things begin to get out of sync.

He mentions the time zone functionality that MySQL offers too, but notes that it cannot be relied on either. Thankfully, there's is a bit of hope in PHP5 (as of 5.1 in fact) - the date_default_timezine_set function. He includes an example of the function in use, making it simple to get the correct date and time automatically with the date() function without having to do any hacks to discover the correct time zone.

On the Builder.com.au website today, there's a quick new tutorial on grabbing the correct time for a user's "home" rather than the server's time.

PHP comes with an extensive catalog of date and time functions, all designed to let you easily retrieve temporal information, massage it into a format you require, and either use it in a calculation or display it to the user. However, if you'd like to do something more complicated, things get much, much hairier

To make things easier, they enlist the service of the PEAR Date package to help bridge the gap between the time zones faster. They give some of the basics of using the package before actually getting into the time zone conversion. The code is pretty straight-forward and they even include other simple tips like how to calculate the GMT offset and how to add and subtract timespans.