The Mayan calendar ends at 2012 because they ran out of room on the rock and were too lazy to re-do it on a bigger rock.
actually, the rock they used was apparently roomier than the one used to develop the Gregorian calendar upon which we lazily hang our proverbial hats...
Length of the year according to modern astronomy : 365,2422 days
Length of our present, corrected Gregorian year : 365,2425 days
Length of the year according to the Maya astronomy : 365,2420 days