The division is often casually referred to as a "month", although it is not coordinated with the lunar cycle.
The Aztec word for moon is metztli, and this word is today to describe these 20-day periods, although as the sixteenth-century missionary and early ethnographer, Diego Durán explained: In ancient times the year was composed of eighteen months, and thus it was observed by these Indian people.
The dates in the chart are from the early eyewitnesses, Diego Durán and Bernardino de Sahagún.
Sahagún's date precedes the Durán's observations by several decades and is believed to be more recent to the Aztec surrender to the Spanish.
Both are shown to emphasize the fact that the beginning of the Native new year became non-uniform as a result of an absence of the unifying force of Tenochtitlan after the Mexica defeat.