Apple has officially acknowledged a date bug in iOS 8 or later that could cause your iPhone or iPad to become inoperable after manually setting the date to a certain period of 1970. A fix for the bug is set to arrive in a future software update.

From Apple:

Manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart.

An upcoming software update will prevent this issue from affecting iOS devices. If you have this issue, contact Apple Support.

The bug in question causes your iPhone or iPad to become inoperable when you preform a reboot of the device after manually setting the date to May 1970 or earlier. It also made it impossible to place your device in DFU or Recovery mode, meaning that you'd need to take it in for repairs in order to get it replaced.

Source: Apple