Why the Maya had three calendars

Now that we're in the year 2012, there will be a flurry of references to the Mayan calendar.  The modern extrapolations of the "long calendar" to a doomsday are of course bunk, but the Mayan calendars are, in and of themselves, fascinating.  I learned a bit about their calenders while reading Charles Mann's book 1419 (reviewed above).

Strikingly, Mesoamerican societies developed three calendars: a 365-day secular calendar like the contemporary calendar; a 260-day sacred calendar that was like no other calendar on earth; and the equally unique Long Count, a one-by-one tally of the days since a fixed starting point thousands of years ago...

Dates were typically given in both notations... Because the two calendars do not have the same number of days, they are not synchronized; the next time 2 Lamat occurs in the sacred calendar, it will be paired with a different day inthe secular calendar.  After October 12, 2004, in fact, 2 Lamat and 11 Yax will not coincide again for another 18,980 days, about fifty-two years.

Mesoamerican cultures understood all this, and realized that by citing dates with both alendars they were able to identify every day in this fifty-two-year period uniquely.  What they couldn't do was distinguish one fifty-two-year period from another.  It was as if the Christain calendar referred to  the year only as, say '04 - one would then be unable to distinguish between 1904, 2004, and 2104.  To prevent confusion, Mesoamerican societies created the third calendar, the Long Count.  The Long Count traces time from a starting point... generally calculated to have been August 13, 3114 B.C... Long Count dates consisted of the number of days, 20-day "months," 360-day "years," 7,200-day "decades," and 144,000-day "millennia" since the starting point.  Archaeologists generally render these as a series of five numbers separated by dots, in the manner of Internet Protocol addresses.  Using the August 13 starting date, October 12, 2006 would be written in the Long Count as 12.19.13.12.18.