Here's my take on it.
It's supposed to be December 21st, 2012, right? But that's on the Mayan calendar, which currently isn't being used by anyone. December 21st on one form of calendar is a completely different day on another, for example, Gregorian and Julian calendars. This date could've passed on the Gregorian calendar, or it could be 100 years from now, who knows?
Either that or they just ran out of rocks to carve.