First: The Spanish simply killed off the Mayans before they could finish their calendar.
isn't there more than one?
I always thought, (from what I had read) that the calendar is finished, it accurately predicts all days tied to celestial events right up until December 21st 2012.
or as the Mayans would have seen it,
winter solstice, (because it's unlikely that they would have known it as december, not least because they would have unlikely had dec (10) as their twelth month of the year, but also because they would have more likely (as astronomers) counted a 13 month lunar calendar) in whatever year they would have counted.
(which I don't know as I don't know when their last "age" started.)
of course, I might be wrong, the Mayans may have predicted the end of the world.
or they could have predicted an interesting planetary alignment that modern day astronomers also predict for dec 21st 2012.
also the Mayans didn't need to account for leap years, as their calendar wasn't a Gregorian calendar,
they didn't say in x days time this event will happen. they said in x days time this celestial event will happen.
where each celestial event is accurately tied to another celestial event.
in fact we add leap days in so that our calendar doesn't stray from these celestial events, (e.g. june 21st is summer solstice, dec 21 is winter solstice.)
In the same way that given the orbit of Jupiter, and the earths orbit, rotation, spin and wobble modern astronomers can say where Jupiter will be in the sky at a given time on a given day. so could the Mayans.