There’s much-spirited commentary throughout the web about changing the date on your iPhone back to 1/1/1970. Doing so will cripple your iPhone so, before we go any further, do not modify the date.
This alert from Apple, while not detailing the cause, expands it to include other products running its iOS software.
If you changed the date to May 1970 or earlier and can’t restart your iPhone, iPad, or iPod touch.
So what’s causing the problem? The short answer is that setting the date back to the beginning of “Unix” time on iOS messes with its 64-bit infrastructure causing an integer overflow thus impairing, or “bricking”, your phone. This video does a better job of explaining what may be going on: