Mesoamerican cultures shared a common calendric system based on multiple intertwining cycles. Its main elements were the Sacred Year of 260 days, called Tonalpohualli by the Aztecs, and a Solar Year of 365 days, called Xihuitl.

The Sacred Year consisted of 13 cycles of 20 sacred named days (referred to as trecenas, based on the Spanish word for 13). Each day was named by combining a named day with a number from the numerical cycle of 1 to 13. Each of the cultures had its own names for the 20 named days, but this system of meshing the two cycles was common to most. The table above shows the cycle of days using Maya names. The far left column lists the 20 named days in order from top to bottom. The columns at the right list the repeating counts of 13 that are combined with the named days. As you follow the columns from left to right, the unique combinations continue in sequence until the 260th day, when the two cycles reach their ends simultaneously (13 cycles of the 20 named days, and 20 cycles of the count of 13). Each combination of named day and number occurs only once in the Sacred Year. Specific gods were associated with each named day, and the influence of these gods was considered in making predictions using almanacs.

The Solar Year calendar of 365 days consists of 18 counts of 20 days (referred to as veintenas, based on the Spanish word for 20), plus 5 days at the end of the year that were considered ill-omened. These 18 veintenas were akin to our months, and each had a name. There are no indications of a "leap" year having been used, but evidence exists that other forms of compensation for the differential were used.

The Sacred and Solar Year dates combined to produce unique day designations, each of which occurred only once in a cycle of 52 solar years (18,980 days). The illustration at above right shows the Sacred Year and the Solar Year as intermeshing wheels. The two wheels at left represent the interweaving cycles of the Sacred Year: the 20 named days are the outer wheel, and the count of 13 is the inner wheel. The numerals are represented by combinations of bars representing 5 and dots representing 1 (i.e., 2 bars and 3 dots represent 13). At right appears a portion of a large wheel representing the Solar Year.

Scholars believe that pre-conquest Mesoamerican cultures conceived of time as circular. They therefore thought they could predict the future by recording events from the past. Using their calendric system and mathematics, they could look both back in time to when they believed the world began, and infinitely forward.