To complete its historical value only one element need be added, the selection of an era from which to reckon dates. Two such eras are known, one bearing the name 4 Ahau 8 Cumhu, and the other (found in only two inscriptions) that of 4 Ahau 8 Zotz, this falling thirteen cycles earlier than the other. The former, from which nearly all the monumental inscriptions are reckoned, is some three thousand years anterior to the period of the inscriptions themselves and probably, therefore, refers to an event in the third millennium B. C., assuming that the monuments belong to the first thousand years of our era. It is altogether unlikely that a date so remote can represent any but a mythical event, such, we may suppose, as the end of a preceding "Sun," or Age of the World, and the beginning of that in which we live; for the Maya, like the Nahua, possessed the myth of ages of this type. Cogolludo mentions two of these ages as terminated by annihilation of the human race through epidemic, and a third as ended by storm and flood; while Landa's account of the calamities following the destruction of Mayapan seems clearly to be intermingled with a myth of world catastrophes. The _Popul Vuh_ shows that the character of the Quiché legend was not essentially unlike that of the Aztec, who may, indeed, have received from the Maya their cosmogony along with their calendric system, of which it is doubtless in some degree a product.
Astronomical data must have entered into the calculation of these great epochs. Förstemann and other students have discovered in the codices, particularly in the Dresden Codex, evidences of the reckoning of the period not only of Venus (five hundred and eighty-four days), but also of lunar revolutions, of the period of Mars (seven hundred and eighty days), and possibly of the cycles Jupiter, Saturn, and Mercury as well. Such periods, for astrological and divinatory purposes, were recorded in the books of the priests; and, as elsewhere in the world, the synodic revolutions of the planets, and the recurrences of their stations with respect to the day-signs, gave the material for the formation of huge cycles of time which their mathematical system enabled them to compute. Thus it is that Förstemann finds near the end of the Dresden Codex vast numbers--designated as "Serpent Numbers" because of the occurrence of the serpent-symbol in connexion with them--which correspond to such cyclic recombinations of signs and events.