You can use GeoIP based services http://www.php.net/manual/en/book.geoip.php
Or
You can use online services that gives timezone of the IP that you supply in request
i.e.
$tags = get_meta_tags('http://www.geobytes.com/IpLocator.htm?GetLocation&template=php3.txt&IpAddress=94.55.180.139');
echo $tags['timezone'];