Though the ancient Maya had been using the Period Ending version of the Long Count they eventually shortened it further by the Postclassic period. This even shorter version does not count a group of katuns like Period-Ending. The Short Count -- whose dates are called katun-ending dates -- only stated the date a katun ended and used a very simplified version of the Tzolkin. This second version was a cycle that didn’t repeat until 256.25 years had passed.
Instead of the regular way of counting the Tzolkin, days were counted using the numbers 1 through 13. Each day had the name Ahau, and was paired with a number between 1 through 13. But it wasn’t just a simple matter of successively counting higher to 13 and starting over. Each day skipped two numbers in the sequence.
An example of the way the Short Count’s dates are written goes like this: katun 13 Ahau. The next katun would be written katun 11 Ahau, after that would be katun 9 Ahau and so on. Once you get to katun 1 Ahau you count down two to get katun 12 Ahau, then 10 Ahau, et cetera.
The Short Count is the kind of time keeping that the authors of books like the Chilam Balam used. In these books, the katun date would be recorded in the u cahlay katunob or the “count of the katuns”. Katuns were named after the last day that they fell on.
“The Ancient Maya”; Robert J. Sharer, Loa P. Traxler; 2006