Recently, a bug was discovered that bricked iPhones if the date was set to May 1970 or earlier. Apple is rolling out a fix with the release of the iOS 9.3 beta.
This update should fix bricked iPhones, using iTunes to restore the affected handset and letting it boot back up to its original state. The update also removes the ability to set an iPhone’s date to a point before January 1, 2001.
Unfortunately for some of those affected by the bug, the iOS 9.3 beta is only available to developers and public beta testers. It should be rolled out to more users in the spring.
If you have been hit by this issue and don’t have access to the beta, Apple recommends that you visit an Apple Store for a replacement handset.