If you have been reading the news recently, you will have read that changing the date on your iPhone to the 1st January 1970 will brick your phone. DO NOT TRY THIS.
Luckily the solution is simple and involves disconnecting the iPhone battery. To do this you will need a special pentalobe screwdriver and to follow the instructions from JerryRigEverything.
His video shows how to resolve this little issue in his 90 seconds video. Of course the easiest solution is not to be an idiot and try and change the date to the 1st January 1970 in the first place.
Apple will be releasing a software update to prevent this happening in due course.