It just occurred to me...
if the cycle is changing, and they keep their calendar on those monster stone disc things... who rolls the old one away and brings in the new one?
I think this is a serious deficiency in stone calendars, and should be investigated.
Firstly, to dispel some ignorance, the often depicted circular calendar stone is an Aztec work, not a Maya one.
The start date of 4 Ahau 8 Cumku comes from two parts, the 260 day cycle, and a 365 day calendar
20 Day names
Imix, Ik, Akbal, Kan, Chicchan, Cimi, Manik, Lamat, Muluc, Oc, Chuen, Eb, Ben, Ix, Men, Cib, Caban, Etznab, Cauac, Ahau
These 20 day names are numbered in tandem with 1 to 13, so 1 Imix, 1 Ik...., 13 Ben, 1 Ix, 2 Men, 3 Cib...., 7 Ahau, 8 Imix, 9 Ik, ...., 13 Cimi, 1 Manik, 2 Lamat, .... 12 Cauac, 13 Ahau.
There are 19 months, 18 of which have 20 days numbered 0 to 19 inclusively, and 1 unluck five day month called Uayeb numbered 0 to 4.
Pop, Uo, Zip, Zotz, Tzec, Xul, Yaxkin, Mol, Chen, Yax, Sac, Ceh, Mac, Kankin, Muan, Pax, Kayab, Cumku, Uayeb.
So, 8 Cumku is the ninth day of the 18th month of the 365 day year, meaning there are 16 days until the end of that particular 365 day period.
As mentioned above 13.0.0.0.0 is 1872000 days from that date 4 Ahau 8 Cumku. So, sixteen days after the start date, we began a new 365 day period beginning with the day called 0 Pop, or more accurately 7 Cib 0 Pop of the current era. If you remove all the 365 day periods you get a left over of (1872000 - 16 - [5128x365]) = 264 days. On the 365 day year, that is the 4th day of the 14th month [(13monthsx20 = 260) + (4 days of the new month, numbered 0, 1, 2, 3)] or the 4th day of the month called Kankin.
This gives us the terminal date of the current cycle called 4 Ahau 3 Kankin, which according to the correlation tables that someone has worked out, falls on 21st December 2012. 4 Ahau will be the same for each 13 baktun block as 1872000 is divisible by 260 without remainder (7200x260=1872000).
Using the dual calendars, and the numbering system, 13.0.0.0.0 4 Ahau 3 Kankin pinpoints a date within 5128+ solar years. This calendrical system is based upon the count of days. However, since the year is longer than 365 days, there is the problem of the accumulation of the year length error. We have sorted our western calendar by using the extra leap day every fourth year or so. The Maya did not adjust their system to account for it.
By the time the Aztec were using a form of meso-american calendar, they didn't utilise the long count, so their calendar was only accurate within an approximately 51.25 year span. This is what the stone calendar disc represents.
