Joseph T. Goodman successfully deciphered the complicated system of the Maya calendar. He published his results in 1897, describing a “Long Count” system of a “count of days” based on several units or periods of increasingly larger size: the k'in (1 day), winal (20 days), tun (360 days), k'atun (7200 days), and bak'tun (144,000 days). The ancient Maya kept track of time using this system, which was combined with additional counts of 260 days (the tzolk'in) and 365 days (the haab) to produce Long Count dates. Goodman believed there was also a larger “Great Cycle” of 13 bak'tuns (1,872,000 days) and determined that the start of the present Great Cycle was on 13.0.0.0.0 4 Ajaw 8 Cumk'u (that is 13 bak'tunob, 0 k'atunob, 0 tunob, 0 winalob, and 0 k'inob, followed by counts on the tzolk'in and haab). Later scholarship showed that this was a sacred “Creation” date for the ancient Maya, who referred to it in their mythology as a kind of “birth” of the present world. The Gregorian equivalent of this date is August 11, 3114 BCE. The next day was 0.0.0.0.1, with each day clicking another unit in the count. According to scholars who support Goodman’s idea of a 13-bak'tun Great Cycle, the current period will conclude on 13.0.0.0.0 4 Ajaw 3 K'ank'in, the Gregorian equivalent of which is December 21, 2012 (or possibly December 23, or yet something else…)
http://www.psychologytoday.com/blog/reality-check/201112/what-you-should-know-about–2012-answers–13-questions