I don’t want to be too alarmist, but try running the following PHP code on your system:

<?php $date = '2040-02-01'; $format = 'l d F Y H:i'; $mydate1 = strtotime($date); echo '<p>', date($format, $mydate1), '</p>'; ?>

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!

What’s 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 represent 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.

Yes, it’s 28 years away and I’m sure many of you think it’s ridiculous to worry about it now. However, developers thought that way about the Millennium bug the 1970’s and 80’s. Also, any web application which handles long-term future events could be at risk. For example, a typical mortgage runs for 25 years. Pensions and savings plans can be far longer.

Will 64-bit save us?

Probably. If you’re using a 64-bit OS with a compiled 64-bit edition of PHP, your application shouldn’t be affected. I’d recommend you test it, though. A signed 64-bit number gives a maximum future date which is 21 times greater than the current age of the universe — 292 billion years, give or take a day or two.

You can probably sleep at night if you’re convinced your financial application will always be installed on a 64-bit system.

Are there alternative options?

Fortunately, PHP introduced a new DateTime class in version 5.2 (experimental support was available in 5.1 and be aware that some methods were introduced in 5.3)…

<?php $date = '2040-02-01'; $format = 'l j F Y H:i'; $mydate2 = new DateTime($date); echo '<p>', $mydate2->format($format), '</p>'; ?>

DateTime does not suffer from Y2K38 problems and will happily handle dates up to December 31, 9999. I might have paid off my mortgage by then!

It may not be worth upgrading existing applications, but you should certainly consider the DateTime class when planning your next project.

Has you experienced Y2K38 problems in your application? How did you fix it?