GPS bug will cause problems with email, iCloud. Check to see that you are running iOS 10.3.4 on iPhone 5 or certain iPads, or 9.3.6 on iPhone 4S and certain other iPads including original iPad Mini. List below.

Owners of older iPhones and iPads must perform a crucial update by Sunday, or they could be stuck with an inoperable device that requires a full restore from the last backup.

The GPS bug does not affect Wi-Fi only iPads. Any device running iOS versions 11 or later is not affected.

Apple alerted owners about the upgrade this week. It affects:

iPhone 5 or a fourth-generation iPad (WiFi+cellular) running iOS older than 10.3.4;

iPhone 4S;

third-generation iPad (WiFi+cellular);

second-generation iPad (WiFi+cellular, CDMA only);

first-generation iPad Mini (WiFi+cellular) running iOS older than 9.3.6.

The update is needed, according to radio station KJCE, "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."

The GPS time rollover issue first was discovered in April and has affected various manufacturers' products.

Without the update, the iPhone 5 will essentially become useless, with email and many apps nonfunctional.

It won't be quite as bad on the other Apple products, with only GPS affected.

The iPhone 5 was last made in 2013.