Here is some more information on what Swift said. The calendar round, and its relationship to the long count and other Mesoamerican cultures, seems to be overlooked whenever the 2012 date is brought up. The calendar round was used by cultures throughout Mesoamerica, including the Aztecs, Zapotecs, and Mixtecs. It is still used today by some Mayans in Chiapas and parts of Guatamala. The unique part of the calendar round is that it repeats every 52 years.

The calendar round is made up of the tzolk'in and the ja'ab'. The tzolk'in has a number and a day and the ja'ab' has a number and a month. The are 18 months of 20 days and one 5 day month (wayeb). The tzolk'in has 20 days and 13 numbers, so it repeats every 260 days. Here is an example how it works.

• 4 Ahaw 8 K'umku
  
  5 Imix 9 K'umku
  6 Ik' 10 K'umku
  7 Akb'al 11 K'umku
  8 K'an 12 K'umku
  9 Chikchan 13 K'umku
  10 Kimi 14 K'umku
  11 Manik' 15 K'umku
  12 Lamat 16 K'umku
  13 Muluk 17 K'umku
  1 Ok 18 K'umku
  2 Chuwen 19 K'umku
  3 Eb' 0 Wayeb
  4 B'en 1 Wayeb
  5 Ix 2 Wayeb
  6 Men 3 Wayeb
  7 Kib' 4 Wayeb
  8 Kab'an 0 Pohp
  9 Etz'nab' 1 Pohp
  10 Kawak 2 Pohp
  11 Ahaw 3 Pohp

I chose 4 Ahaw 8 K'umku because that is the calendar round day on which the Mayan long count begins at 0.0.0.0.0 and it is very important to them. However, as I said before, the calendar round repeats every 52 years (in this case 1 year is exactly 365 days) and the special date of 4 Ahaw 8 Kumk'u does not fall on a bak'tun rollover, like 13.0.0.0.0, until the long count date of 2.7.9.0.0.0.0. For those of you who are worried about that date, you can rest easy, since it won't come until January 12, 371039 (GMT Correlation).

• The 4 Ahaw 8 Kumk'u dates around 13.0.0.0.0 are
  12.15.14.1.0 April 6, 1928
  12.18.6.14.0 March 24, 1980
  13.0.19.9.0 March 11, 2032
  13.3.12.4.0 February 27, 2084

It seems strange that the 52 year calendar round, which was so important that it was used throughout Mesoamerica, doesn't even complete a full cycle at the supposed end of the world.

The Lunar Series was probably the most accurate calendar used by the Mayans, since it was adjusted to keep it in step with lunar phases. The Ja'ab' calendar had no leap year. I'm not quite sure how it worked, but it contained several glyphs. One told the age of the moon from the last new moon, another told the name of the lunar month (there were six total), and the last gave the expected length of the current lunar month (either 29 or 30 days). There were also other systems used in the calendar, including the "Lord of the Night" series, the 819 day cycle, and a 7 day cycle. The 819 day cycle also had an associated direction and color.

Just for fun, using the GMT correlation, April 5, 2004, is

• 12.19.11.2.18 7 Etz'nab 1 Pohp
  Lord of the Night - G4
  Short Count - 4 Ahaw
  819 Day Cycle - 1.16.2
  Direction - South
  Color - Yellow

Scott