The retro iPhone hoax has been floating around the internet for a while now. It claims that if you set the date on your iPhone to the 1st of January 1970, you will be rewarded with a retro iPhone theme.
What actually happens is that your phone will get stuck in reboot. This is irreparable. It seems to affect any device with a 64bit processor running iOS. So, the iPhone 5s onwards, iPad Air and iPad Mini 2. Apple has confirmed this issue but as of yet, there is no solution, save replacing the phone. An upcoming software update should fix the issue.