Mayan Zodiac Sign Calculator

A Mayan day sign is one of the twenty named days of the Tzolkin, the 260 day count that runs twenty day signs against the numbers 1 to 13. Every date carries exactly one pairing of a sign and a coefficient, and the pair repeats only after 260 days. In the living highland tradition the sign is called a nawal, and the day someone is born on is read as the character they carry.

The date is converted to a day number, and that day number is divided against the two Tzolkin cycles: twenty for the sign and thirteen for the coefficient. The tie between a day number and a civil date is set by a correlation constant, and this calculator states the constant it used on every result, so the same date always returns the same day sign and a saved reading stays reproducible.

Two reasons, and both are input conventions rather than arithmetic. First, published correlation constants sit two to three days apart, so a tool running a different constant shifts every day sign by that many days. Second, dates here are proleptic Gregorian, extended backwards unchanged through the 1582 reform, while some converters switch to the Julian calendar below it and disagree by ten or eleven days on pre-reform dates. Calendars that renumber the days on a fixed 365 day year drift from the classical count entirely.

A trecena is a run of thirteen consecutive days in the Tzolkin, and there are twenty of them in each 260 day round. The sign of the first day rules the whole run, so a reading names both the day sign itself and the trecena it sits inside. Daykeepers read a run of days as a unit, which is why the trecena and the position within it are returned beside the sign.