A recently-uncovered date glitch in iOS, which has evidently been present since, iOS 8, will cause your phone to become inoperable if you reboot your device after manually setting the date to January 1, 1970. You will also apparently be unable to place your phone in DFU or Recovery mode, so a Genius Bar appointment may be in order if you do this.
The glitch surfaced thanks to a video from Zach Straley on YouTube.
In order to avoid this, simply let your iPhone automatically set the time. If for some reason you need to manually set the time, don't set it to January 1, 1970. Don't under any circumstances, follow any instructions that tell you to do so. And as always, remember to exercise a little common sense.