The Mayan calender predicts the end in 2012. How has this be established they were before our calender started

how did the mayans's make the dooms day prediction as 2012? I heard recently that in 2012 we will cross as spacial distortion that will cause the poles to swap (would this lead to floods?) I'm guessing that they were just really good astrologers or astro physicists, however as their calander and our calander are different, who and how was the date 2012 reached? how accurate is that date?