There's a rumor running around that if you re-set the date on your iPhone to January 1, 1970 and reboot, you will unlock a secret "Retro Mode".  Allegedly, it has an old rainbow-colored Apple logo and some other junk, but there's a couple of problems.

First, it's a HOAX.  A 1970 retro mode wouldn't even make sense because Apple wasn't founded until 1976.

Second, you'll permanently destroy your phone if you do it.

Setting the date back that far screws up the software in your phone and crashes it – for good.

They MIGHT be able to fix it at the Apple Store, but more than likely, you'll end up having to buy a new one.

The good news is, it's kind of tough to set your phone back that far because of a lot of long, boring scrolling.  So you can't really do it accidentally.