Here’s a PHP class I wrote way back when, which can be used for shifting datetimes. It is also hosted at PHPClasses, but I don’t like how you have to register there to access the code. Here it is in bzip2 format if you want to download it:

date_time_manager.tar.bz2

This code comes with NO WARRANTY, and I have no idea if it works with PHP5. I did find that someone is actually using it though, which is awesome!

http://www.care2x.org/home_legal.php