An iPhone bug was discovered recently that bricks phones if the handset's date is set to 1970. Apple has acknowledged that this bug exists and says that a fix will be included in a future software update.
Apple updated its support page to say, "Manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart." A future software update will fix the problem, but Apple hasn't indicated when this will land.
You may wonder why someone would change their handset's date to sometime in the 1970s, but it could have been out of curiosity, because they believed someone who said that something cool would happen, or someone may have changed the date on them. Regardless, if this has happened to you, you're advised to contact Apple Support.