They ran out of rock to put it on. They had to stop somewhere didn't they, or else they'd still be at it. Right now they'd be hacking out the year 5022 and someone would be asking if THAT would be the end of the world then.
December 21 st ?? Puts fear into people. BUT ONLY God can predict the end of time. Our bible says the end will come when the gospel is preached unto all the world because God does not want any to perish.