About the Calendar Correlation This plugin uses the Alfonso Caso – Nicholson correlation, the most widely accepted scholarly standard for converting Gregorian dates to the Aztec calendar. This correlation is anchored to the historically documented date of August 13, 1521 (the fall of Tenochtitlan), which corresponds to the day 1-Cóatl (Serpent) in the Aztec tonalpohualli.

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