Apple has acknowledged that switching your iPhone's date back to May 1970, or earlier in that year, will brick your phone beyond use - and it is working on a fix.
A number of users were fooled into switching their device back to January 1, 1970, after a Reddit post suggested it would give a 70's themed Easter egg, but instead it bricked their devices so they couldn't restart their phones.
Apple has now updated its support page to admit the problem exists. It says: "Manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart."
Apple also said an upcoming software update would bring a solution to the problem, but there's no sign of when it will actually land.
If you've already had the problem affect your phone, Apple's advice is to contact Apple Support.