There is apparently a bug on 64-bit iPhones (5s and newer) where setting the date to January 1, 1970 will put it into a boot loop where it won’t boot. In fact, iTunes will not be able to see it to do any type of restore, essentially bricking the device. (Bricking is a term used for disabling a device to the point where it can’t do anything, turning it into a “brick”.)
After reading through the threads on Reddit, it seems that disconnecting the battery may be able to fix it, but that isn’t guaranteed. Most people are having to have their iPhones replaced by Apple (and if you’re out of warranty, that’s going to cost you).
This is also making the rounds as Blast from the Past.
Don’t fall for the trick!