If you’re using an iPhone or iPad introduced in 2012 or earlier, you should check your device for an iOS update if you want it to keep reporting the correct time, date, and location.

Apple is pushing a software update to a selection of older iOS phones and tablets to fix an issue affecting global GPS. The problem is known as the Week Number Rollover issue, and its caused by GPS systems recording the passage of weeks as a 10-bit figure. This means that when they hit week 1,024 (just under 20 years) their count resets. As a consequence, some older hardware is thrown off course — both figuratively and literally. It’s similar to the Millennium/Y2K bug, which was also caused by the reset of a date count.

Other GPS systems are also affected by the issue

GPS makers like TomTom and Garmin have been issuing patches for this problem for a while, and now Apple is doing the same. The earliest the Week Number Rollover issue started affecting devices was April 6th this year, but Apple says its hardware won’t be affected until November 3rd.

Newer phones and tablets that work with the latest version of iOS (and have been updated!) are already covered, but Apple is now extending that coverage to older devices, too.

The software update will appear as iOS 10.3.4 for the iPhone 5 and fourth-generation iPad, and iOS 9.3.6 for the iPhone 4S, first-gen iPad mini, iPad 2, and third-gen iPad. Apple says the issue doesn’t affect Wi-Fi-only iPads or the iPod touch.

You can read the full announcement from Apple here, with instructions on how to update your phones and tablets. It may sound like an unnecessary hassle, but if you’ve gone to the trouble of keeping your old iPhone or iPad going, it’s worth updating it to keep this important functionality.