You are underestimating how complicated that problem is.  The easiest and most accurate way to do this is to simply have users tell you which timezone they are in and set that in their profile.

If you want to do it automatically you need a huge geoip database to get the ip to a lat/lon and then once you have that you need a huge geo database to turn the lat/lon into a timezone.  We can't possible include these with PHP, and they would be out of date in weeks.  

What we do provide, however, is the base functionality to make it easy to connect to services that have these databases.  For example, we have a geoip extension in pecl. See http://pecl.php.net/geoip and the example of it in action here: http://geoip.pidgets.com
Once you have the lat/lon you can query one of the online geo apis.  Like this one:
http://gws.maps.yahoo.com/FindLocation?q=35,-100&flags=T&gflags=R

That turns lat/lon 35,-100 into a timezone (among other things).  But, a geoip lookup is no usually very accurate.  And in some cases wildly inaccurate.

If you're in a one-timezone-only country such as the UK, you can simply do:

<?php
$ids = DateTimezone::listIdentifiers( DateTimeZone::PER_COUNTRY, 'GB' );
echo date_create( $ids[0] )->format( DateTime::RFC2822 ), "\n";
?>

http://geonames.org provides a free database that can map cities to timezones as well.