It was recently discovered, and now acknowledged by Apple, that setting your iOS device to a date prior to May 1970 will permantently brick it.
The dreaded "1970 date" bug was officially acknowledged by Apple yesterday. For whatever reason, manually changing the date to 1970 or earlier can prevent your iOS device (iPhone, iPad, or iPod touch) from turning on after a reset, the company said on its support website.
Apple assures an upcoming software update will prevent this issue from affecting your iOS devices, but for now, you must contact Apple Support if you are having this issue. In the meantime, don't set your date to May 1970 or before.