Changing the date of your iPhone to January 1st, 1970 will brick your
device, according to the report on Reddit. The bug will affect all the devices with 64-bit
processor like A7, A8, A8X, A9, and A9X but 32-bit iOS devices are reportedly
not affected by this issue. 

Changing the date of your iPhone to January 1st, 1970 will brick your device, according to the report on Reddit.

Once you set your date to January 1st, 1970, it will permanently brick your
iPhone and stuck in an Apple logo and cannot restore your device either from
DFU mode or Recovery mode. Though the restore will succeed, but the device
will still not boot after restore. So don’t try this bug on your own
handset.

A physical repair (either a Genius Bar appointment or physically
disconnecting battery and connecting again) is required for the device to
function again.
To change your date and time settings:
Go to the Settings app > General > Choose Date & Time >Turn
off the Set Automatically toggle. Scroll down until you reach January 1st, 1970. The video below shows how to do it
and how it bricks the 64 bit processor devices.
After seeing this issue, Apple will soon release a software update to
resolve the issue impacted by this bug. 
Source: Reddit