As an introduction to the Datetime class functionality PHP 5 has, Kevin Waterson shows how to use it to calculate the "Friday the 13th" for any year.
This class is a request from a PHPRO.ORG regular who asked for a method to calculate Friday the 13th for the next n years. The calculation itself is easy, however, it presents a good opportunity to introduce the PHP datetime class.
He points out that timestamps, because of they way integers are handled, have a somewhat limited lifespan (2038) and dates beyond that cannot be determined by the usual date functions. Instead, he shows how to set up a DateTime object, set a timezone and loop through the days to find the Fridays that land on the 13th.