The existence of Mesoamerican calendars is known as early as ca. 500 BCE, with the essentials already appearing fully defined and functional. These calendars are still used today in the Guatemalan highlands, Veracruz, Oaxaca and Chiapas, Mexico.
Among the various calendar systems in use, two were particularly central and widespread across Mesoamerica. Common to all recorded Mesoamerican cultures, and the most important, was the 260-day calendar, a ritual calendar with no confirmed correlation to astronomical or agricultural cycles. Apparently the earliest Mesoamerican calendar to be developed, it was known by a variety of local terms, and its named components and the glyphs used to depict them were similarly culture-specific. However, it is clear that this calendar functioned in essentially the same way across cultures, and down through the chronological periods it was maintained.
The second of the major calendars was one representing a 365-day period approximating the tropical year, known sometimes as the "vague year". Because it was an approximation, over time the seasons and the true tropical year gradually "wandered" with respect to this calendar, owing to the accumulation of the differences in length. There is little hard evidence to suggest that the ancient Mesoamericans used any intercalary days to bring their calendar back into alignment. However, there is evidence to show Mesoamericans were aware of this gradual shifting, which they accounted for in other ways without amending the calendar itself.
These two 260- and 365-day calendars could also be synchronised to generate the Calendar Round, a period of 18980 days or approximately 52 years. The completion and observance of this Calendar Round sequence was of ritual significance to a number of Mesoamerican cultures.
A third major calendar form known as the Long Count is found in the inscriptions of several Mesoamerican cultures, most famously those of the Maya civilization who developed it to its fullest extent during the Classic period (ca. 200–900 CE). The Long Count provided the ability to uniquely identify days over a much longer period of time, by combining a sequence of day-counts or cycles of increasing length, calculated or set from a particular date in the mythical past. Most commonly, five such higher-order cycles in a modified vigesimal (base-20) count were used.
The use of Mesoamerican calendrics is one of the cultural traits that Paul Kirchoff used in his original formulation to define Mesoamerica as a culture area. Therefore, the use of Mesoamerican calendars is specific to Mesoamerica and is not found outside its boundaries.
In the 260-day cycle 20 day names pairs with 13 day numbers, totalling a cycle of 260 days. This cycle was used for divination purposes to foretell lucky and unlucky days. The date of birth was also used to give names to both humans and gods in many Mesoamerican cultures; some cultures used only the calendar name whereas others combined it with a given name. Each day sign was presided over by a god and many had associations with specific natural phenomena.