Brandon Savage has posted about some interesting things he's come across when working with the DateTime objects in PHP 5.3 including working with timestamps and differences in terms of days/months/etc.
Over time, the PHP DateTime object has become one of the best objects available to PHP developers. This object has grown since early PHP 5 into a robust class that has the ability to do lots of great things. Recently, I was exploring some of the functionality provided by the DateTime object as of PHP 5.3 (and wishing that Ubuntu had PHP 5.3 as a package distribution). Here are some of the new things in PHP 5.3 that are really cool.
He mentions the "add()" and "sub()" methods you can use on an object to get information about the dates/times forward and backwards, the "diff()" to get the differences between two dates and the get/setTimestamp methods to get the current time (or set it).