A bug in iOS 8 and later can cause iOS devices with 64-bit socs to enter a bootloop. By scrolling back to January 1, 1970 when setting the date and then rebooting the device, iOS can no longer boot properly.
Scrolling back to January 1, 1970 takes a few steps, because iOS doesn’t go back that far in one go. If you want to, scroll back as far as possible, then click Back, then set the date again and go back further and further to January 1, 1970. After that, the device will work, unless the user decides to reboot. The bug surfaced in posts on 4Chan and Reddit.
It is unknown what causes the bootloop, but it is possible that iOS works with a Unix timestamp, which assigns the number 0 to midnight GMT January 1, 1970. If users are in a time zone west of this time zone, it would be a ‘negative time’. Apple has not yet confirmed the bug.
Users on 4Chan and Reddit point to various fixes. Since restoring iPhone in dfu mode doesn’t work, some users suggest to put a SIM card in the device after restore. Also draining the battery completely seems to work. Some users have successfully invoked the warranty and obtained a replacement device from Apple.
The bug seems to only affect devices with a 64bit soc. This includes the iPhone 5s and newer and the iPad Air and newer. Zach Straley, among others, has posted a demonstration of how the bug works on YouTube.