A baktun (properly b'ak'tun) is 20 katun cycles of the ancient Maya Long Count Calendar. It contains 144,000 days, equal to 394.26 tropical years. The Classic period of Maya civilization occurred during the 8th and 9th baktuns of the current calendrical cycle. The current (13th) baktun will end, or be completed, on 13.0.0.0.0 (December 21, 2012 using the GMT correlation). This also marks the beginning of the 14th baktun, as such a term is usually used among Mayanists.
J. Eric S. Thompson stated that when a Long Count of, say, 9.15.10.0.0 is placed in the 9th baktun, we are almost certainly committing an error, like placing the year 2009 in the 2nd millennium. However, that practice is so well established among Maya epigraphers and other students of the Maya that to change it would cause more harm than its perpetuation.[1]