Yeah, essentially you're assuming that we've looked at the calendar, and said 'it takes us through to this day, divide that into years and you get this date'. it's not how it works... what the calendar said was 'when the stars/moon/etc. are aligned in this position (which corresponded to the dating system at the time) the calendar ends' which corresponds (accounting for leap years) to december 2012. you can't think of the mayan calendar as a big list of days, that's not really how it works (from what I hear). it's a calendar in the most traditional sense, in that it charts astronomical phenomena, and this doesn't really fit to a calendar in the sense that we see it today, so the leap year thing doesn't matter.
I explained that appallingly, but I hope you'll get the gist of what i'm saying...