They seem to have run out of stone while making their calendar (considering that they had enough stone to chart out 2000 odd years after their time!), and it stops on December 12, 2012.

And it was he, without doubt, who scratched a calendar on this stone.

And every month when it came to the evening of the 15th of lunar calendar, we would climb onto the stone, looking up at the sky, hoping to see the full moon come out from far away.