Apple is having a bit of a "Y2K" issue with its iOS devices. Last week, it came to light that setting the system date of iPhones, iPads or iPod touches to January 1, 1970 basically renders them useless. Apple Support confirmed the bug on Monday, stating that "manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart."
Apple is working on fixing the problem in its next software update, but there's speculation that it to stems from iOS being a Unix-based system (Unix time starts at 0:0:0 UTC on January 1, 1970). Apple probably didn't expect users to turn the clocks on their iPhones back that far (because why would you?), so their developers now have to waste their time fixing a problem that really shouldn't be a problem at all.
Apple's next iOS update is scheduled for March, but it wouldn't be surprising if they pushed through a fix for the bug sooner, as Apple Store employees are probably tired of dealing with the folks who have inadvertently given their phone a brain freeze.