There's an iOS bug where users who set their device's date to 1970 or earlier end up with a bricked device or at least an iPhone or iPad that won't boot up until the battery dies. A fix in an upcoming software update will fix the bug which will render devices useless. Below is Apple's statement on the matter.
Apple’s comment on the situation is short, but it claims that the bug can cause iPhone crashing with any date before May 1970.
Manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart.
Source: Apple