On the XpertDeveloper.com site there's a post reminding you of an date could cause all sorts of problems with your PHP application - the effects of the Y2K38 bug.

Y2K38, or the Unix Millennium Bug, affects PHP and many other languages and systems which use a signed 32-bit integer to signify dates as the number of seconds since 00:00:00 UTC on 1 January 1970. The furthest date which can be stored is 03:14:07 UTC on 19 January 2038. Beyond that, the left-most bit is set and the integer becomes a negative decimal number or a time prior to the epoch.

If you're worried about your application's support for date and time handling, there's a pretty simple fix - replace your current handling with the DateTime functionality. This handles them correctly.