Why does iOS 9 get bricked when reset to 1970?
In some time zones, setting the date to 1 Jan 1970 will set the internal clock to a number less than zero, as the time is stored in GMT (as the number of seconds since midnight on that date) and then the offset is applied before display. In other time zones, setting the clock will result in a positive time value. Best guess is that this is triggered by having the time value less than zero.

This behaviour can be replicated by creating an NPT server on a network and injecting it to multiples devices. If the time returned to the device is negative, the device will crash.