It is actually much more simple than that. It's called the astronomic procession. Their calendar was based on how long it would take to go from one fixed point in the heavens all the way around the wheel to return to the start point. They quite rightly assumed that anyone smart enough to understand the first cycle would be able to understand that there were others before, and would be others after. The wheel never stops turning.
There's also the issue that the "ZOMG the Mayan calendar ends in 2012" people either ignore, or are unaware, that the long count calendar includes several time units longer than a b'ak'tun, or that even discounting pik'tuns and longer units, December 21 2012 is not even the "end of" the current era, it's just when we roll over from bak'tun 12 to bak'tun 13 (from 12.19.19.17.19 to 13.0.0.0.0). The current era of the long count does not end until October 12, 4772 (19.19.19.17.19), after which we roll over to 1.0.0.0.0.0 and begin a new pik'tun. Mayan inscriptions have been found referencing dates FAR beyond 2012, some of them staggeringly far beyond. The inscription on Quirigua stela F contains a long count date over 90 million years in the past, and Quirigua stela D references a long count date over 400 million years in the future. A stela at Coba expresses a date (believed to refer somehow to the creation date of Maya myth; I'm not quite clear on the basis for this calculation) of 13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.0.0.0.0, which contains no less than 15 units greater than the alautun, which itself is a period eight thousand times longer than a pik'tun and one hundred and sixty thousand times longer than a bak'tun.
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