Dr Pamela Gray discusses calendars on Astronomy Cast Episode 245 with host Frazier Cain, they relate how our present calendars came to be and (18:00 mark) discuss the Mayan Calendar. In the show notes, an excerpt of article by Griffith Observatory Director, Dr E C Krupp, is included, “the Mayan calendar is not spooling up the thread of time. It is coming to the end of a particular cycle in an unending sequence of cycles. According to the rules of the Maya calendar system, a primary interval, Baktun 13, for all practical purposes ends on the winter solstice, 2012. Although pseudoscientific claims have linked this calendrical curiosity to a Maya prophecy of the end of time, there is no evidence for ancient Maya belief in the world’s end in 2012 or even in any unusual significance to the cycle’s completion.The Maya calendar relied on multiple cycles of time. In Maya tradition, these cycles of time run far into the future, and there are ancient Maya hieroglyphic inscriptions that project time into the future well beyond 21 December 2012. At the end of Baktun 13 (a period of 144,000 days or 394 years), a new baktun will begin. There is no Baktun-13 end of time. The notion of a Baktun-13 transformational end of time is modern. It originated in Mexico Mystique, a book published in 1975 by an American writer, Frank Waters, who made computational errors.” You can read more about coming "rollover" of Mayan calendar in a 2009 Sky and Telescope article by Dr Krupp. An anticipated dismissal of doomsday rumors that surround end of Mayan calendar didn't occur in this Astronomy Cast, Dr. Gray commented, simply, "we don't know."