Some iPhones or iPads released in 2012 or earlier need a software update from Apple, or they will stop working correctly at 2 p.m. eastern time on Sunday, Nov. 3.
In particular, iPhone 5 phones that do not install the iOS 10.3.4 update will no longer be able to connect to online services that require the correct date and time, meaning they might not be able to download apps from the App Store, access email, or browse the web , Apple said on its website.
If you miss the deadline, you won't be able to download the update in the iPhone settings menu, and will need to update by plugging your phone into a computer.
"If you have an iPhone 5, it's especially important to update your device's software wirelessly or using your computer before November 3 to maintain accurate GPS location and to continue to use functions that rely on correct date and time including App Store, iCloud, email, and web browsing," Apple wrote.
iPhone 4s devices and some iPad Mini, iPad 2, and iPad 3rd generation models with a cellular chip need an update to iOS 9.3.6 or they will no longer be able to maintain an accurate GPS position.
The update is necessary because of a "GPS time rollover issue," Apple said on its website. According to Ars Technica, it's a well-known issue that affects many different device and system makers and is related to the system's "week number" clock reaching its limit and flipping back to 000000000.
Most iPhone users probably already have much newer software on their devices: A newer version of iOS is required to sign up for some Apple services and even to connect to new Apple hardware products like AirPods Pro.
iOS 13 was released this fall, and can run on the iPhone 6S and iPhone 6S Plus, which were released in 2015, and all later models. In environmental disclosures, Apple says it assumes three years of use from the first owner of an iPhone.
But there are people out there still using phones from 2012 and earlier, so Apple has released new variants of older iOS versions to keep those devices working and fix the GPS rollover issue. iOS 10.3.4 and 9.3.6 came out in July.
"This issue does not affect iPod touch or any iPad models that have Wi-Fi only. It also doesn't affect iOS devices newer than those listed above," Apple said on its website.
You can update your phone or tablet in Settings > General > Software Update.