I’ve seen a lot of newbies (and not newbies) having problems handling dates in PHP (and even with SQL and another languages). When I see someone having problems with dates, I always ask the same question. I type in a text editor “27/11/2012″ and I ask him: What is it? If your answer is “This is a date” you should continue reading the post.
He talks about how the DateTime functionality replaces (much more effectively) some of the older date handling methods in PHP. He includes a few examples comparing it to date and showing how it can be used to compare dates. He includes a "Dummy" class he mocked up to show how you could work with DateTime to get/set formatted dates, set the format to use and get the current format. As always, he also provides tests for the code as well.
This is just the tip of the iceberg as to what DateTime can do, so I'd suggest checking out the manual page for it to see the full list of features.