If you’ve managed to hold on to your 2012-era iPhone 5 until now, you should probably update it before November 3rd.

You’ll need to update to iOS 10.3.4 in order to keep important functions including the App Store, iCloud, email, and web browsing working, according to Apple. If you’re not sure which version you’re running, Apple has a support page about the issue, which also explains how to check which version you’re using.

Your phone needs the update because of the GPS time rollover issue. Basically, GPS devices measure date and time using a week counter, which is only able to count up to week 1,024, after which the counter resets back to zero. Since GPS requires accurate time data to function, the reset can disrupt location data, too. Date resets are a recurring issue for technology, probably most famously in the Millennium / Y2K bug.

Clocks have stopped Apple products before

Newer devices have been designed to adapt to the rollover, according to Ars Technica, and it might only be your GPS accuracy that gets affected by the bug, if you’re affected at all. Apple says the iPhone 4S, first-generation iPad mini, iPad 2, and fourth- and third-gen iPad will need an update to keep their GPS working accurately, although other functions should continue to work if you forget to update. Verizon said that the GPS accuracy on any device built between 2006 and 2016 could be impacted, and published a list of affected devices here.

Fortunately, for most devices there’s an easy fix: just update.

You can update your phone wirelessly or through your computer before November 3rd. Here are Apple’s instructions on how to do that. If you don’t have the update installed by November 3rd, though, Apple says over-the-air updates and iCloud backup will also no longer work, since the GPS issue affects iCloud. If that happens, you would need to back up and restore your phone using a computer. Here’s another guide on how to do that.

Clocks have stopped Apple products before. A bug caused iPhones to crash multiple times once the clock struck 12:15AM on December 2nd, 2017. Last year, daylight saving time confused Apple Watches, trapping them in a crash and reboot loop.

Apple says the GPS rollover won’t impact their devices until just before 12:00AM UTC on November 3rd, which is 8:00PM ET on November 2nd. But don’t wait until then — go update your phone now!