There is a Reddit post that warns iPhone users with a A7, A8, A8X, A9 and A9X chip and iOS running on 64 bits, not to set the date back to January 1st 1970. If you do that and restart your phone, the device will not boot. Putting your phone in DFU or recovery mode will allow a restore, but the phone will still not boot. The only way to fix it is to take it to an iPhone repair shop. Here is a video how this bug works :
For jailbroken devices there is a fix already. Tweak developer, Ziph0n, already released a fix in his repository repo.ziph0n.com. Add it to your Cydia sources if you didn’t. Download the tweak from here (iOS) : cydia://package/com.ziph0n.brickingdate.