As the Maya were very good astronomers and observers they had a complex series of calendars, including a Sacred 260-day calendar, called the Tzol'kin, a 365-day calendar called the Haab, and a 52-Haab cycle called the Calendar Round, which synchronised the Tzol'kin and Haab cycles.
There was also a Long Count calendar which started at [0.0.0.0.0] (with Maya record) on August 11, 3114 BC according to the "Goodman, Martinez-Hernandez, and Thompson" correlation (nicknamed "GMT"), the most widely accepted correlation between the Maya and Gregorian calendar. This cycle is 1,872,000 days in length, terminates on the Winter Solstice of (December 21) 2012 AD and is designated [18.104.22.168.0] or [0.0.0.0.0], since the Maya believed that time is somehow periodical. Another widely-used correlation, that of Lounsbury, correlates the start-day to August 13, 3114 BC and the terminal date to December 23, 2012 CE.
The turn of the great cycle is conjectured to have been of great significance to the Maya, but does not necessarily mark the end of the world. According to some students of the Maya calendar, the Maya chose the start date for their long count so that the alignment of the Sun with the Galactic Center will occur on the end date, although according to astronomers it is not possible to specify such an "alignment" with a precision of less than a few years.