It's said that the apocalypse will come about.
Apocalypse, however, literally means "Unveiling", so it does not necessarily mean the world imploding.
As suggested in Dan Browns
The Lost Symbol, it will actually mean a global realisation of a great truth and the human race developing almost "godlike-like" capabilities.
Also, does it say anywhere that the world ends, or does the calender simply stop?
As suggested by Cervantes, perhaps they simply ran out of room?
It's kind of like the Large Hedralon Collider. Everyone was running around like headless chickens, but nothing happpened.