Copan Stela D - 10 Ajaw (8 Chen) Calendrics
The Maya had very complex and interlocking calendar systems, which were as precise as modern day calendars. The Maya recorded time mainly using 3 interconnected calendars: the Tzolk'in (260 day count), the Haab (365 day count), and the Long Count. Most Maya dates were expressed as a combination of the Tzolk'in and Haab. This combination is called a Calendar Round.

The Tzolk'in consists of the numbers 1 - 13 alternating against a cycle of 20 day names, with their number-day combination restarting every 260 days (13 x 20 = 260). For example (see figure 1), the iteration starts with the day 1 Imix and proceeds for 13 days to 13 B'en, then continues another 7 days counted from 1 Ix (day 14), 2 Men (day 15), 3 Kib'(day 16), etc, to 7 Ajaw — for a total of 20 days. Then the days will start over again with Imix, but continuing with the number sequence 8, or 8 Imix.
Tzolk'in (20 day names)
Figure 1
1 Imix11 Chuwen
2 Ik'12 Eb'
3 Ak'bal13 Ben
4 K'an1 Ix
5 Chikchan2 Men
6 Kimi'3 Kib'
7 Manik'4 Kab'an
8 Lamat5 Etz'nab'
9 Muluk6 Kawak
10 Ok7 Ajaw

The Haab cycle repeats a series of 18 months of 20 days each (0-19), with an additional month of 5 days at the end (18 x 20 + 5 = 365). The months cycle is very similar to ours, for example (see figure 2) the month Pop begins and counts through 20 days, then goes on to the next month Wo.
Haab (19 month names)
Figure 2
0 Pop (seating of Pop)11 Pop
1 Pop12 Pop
2 Pop13 Pop
3 Pop14 Pop
4 Pop15 Pop
5 Pop16 Pop
6 Pop17 Pop
7 Pop18 Pop
8 Pop19 Pop
9 Pop0 Wo (seating of Wo)
10 Pop1 Wo

Long Count
To keep track of linear time, the Maya created a positional notation system known as the Long Count. This is a number, used similarly to our numerical "year 2008," counting "years" and days since the last Creation in 3114 BC. (The "years" here counted, called Haabs, are only 360 days long.) Each 'digit' of the Long Count is twenty times the next one, just as each digit of 2 0 0 8 is ten times the value of the following. We call our system, based on 10's, decimal notation; while the Maya system is vigesimal, based on 20's. The Long Count system counts in increments of twenty which provide the quantity of the b'aktun (400 years), k'atun (20 years), tun (360 days), winal (months), and k'in (days).

For example, the Gregorian date records:
Monday, December 29th 2008
Monday = One day in a named cycle of 7 days (week)
29th = One in a numbered cycle of 28, 29, 30 or 31 days
December = One in a cycle of 12 named months
363 = One in a cycle of 365 days AD/CE = A count of years since the birth of a Christian cycle
To compare, this same date as written by the Maya records:
7 Manik' 10 K'ank'in
Manik' = One day in a named cycle of 20 days (tzolk'in)
7 = One in a numbered cycle of 13 days
K'ank'in = One in a cycle of 18 named months (haab)
10 = One in a numbered cycle of 20 days = A count of years since the birth of a Maya Cycle

Click on the various links below to learn more about calendrics, how to convert a Gregorian date into a Maya date, and the glyphs associated with Maya dates.
Inga Calvin's Glyph Guide Section 1 - Calendrics
Date Conversions
Mark Van Stone's It's Not the End of the World: What the Ancient Maya Tell Us About 2012
David Bolles explains The Mayan Calendar, The Solar - Agricultural Year, and Correlation Questions
Print Current Month Calendar