The Authentic 260-Day Count

The Tzolk'in, to use the modern Yucatec-based coinage for it, is a cycle of 260 days produced by running the numbers one to thirteen against twenty named days, so that a given number-and-name pair recurs every 260 days. It is the oldest continuously used calendar in the Americas: 260-day counts are attested in Zapotec inscriptions from Monte Alban by the sixth century BCE and are shared across Mesoamerica, appearing among the Mixtec, the Aztec, who called their version the tonalpohualli, and the Maya.

Meshed with the 365-day Haab, it produces the Calendar Round of 18,980 days, about fifty-two years, after which a full date repeats. The origin of the number 260 is debated: proposals include the approximate human gestation period, the interval between zenith passages of the Sun at certain Mesoamerican latitudes, and the product of the ritually significant numbers thirteen and twenty. No single explanation is established.