Correlating Aztec and common datesystems
Some discussion exists on how to correlate the Aztec calendar to our common datesystem. The most supported correlation by historians is the one suggested by Alfonso Caso. An anchor for this correlation is the recorded date of 1-Coatl for August 13, 1521 Julian (the fall of the city of Tenochtitlan). Most other mesoamerican calendars, like for example the Mayan calendar, use a daycount that is in sync with this correlation.
Based on a study of the Codex Borbonicus, Caso concluded that the last veintena (meztli) of the year was Tititl. However, as claimed by H.B. Nicholson, Caso's case is not very clear and many ethnohistorical sources suggest that Izcalli was the last veintena of the year. This alignment of the year ending, in combination with the Caso correlation, is now the default setting used by this site. You may switch back to the original Caso correlation.
The correlation can also be set to that of Francisco Rodriguez Cortes. Francisco is a current day calendrist who lives in Chiapas, Mexico and an exponent of the living tradition of the tonalpohualli.