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.

I don't want to be too alarmist, but try running the [given] PHP code on your system. [...] With luck, you'll see "Wednesday 1 February 2040 00:00" displayed in your browser. If you're seeing a date in the late 60's or early 70's, your PHP application may be at risk from the Y2K38 bug!

The bug, caused by a 32-bit operating system, can be helped by running the application on a 64-bit platform (it's due to the limitation of integer size), but there is another option - the DateTime class that handles dates and times differently than the just using the local time settings.