Aztec calendar

The Aztec calendar was a human dating system that cycled every 52 years over a period known as a xiuhmolpilli, leading to the New Fire Ceremony, where the years must be bound through ritualistic sacrifice to renew time. (PROSE: Against Nature) The calendar had a pair of sub-calendars serving dual purposes, one for tracking the solar year, and the tonalpohualli, which had religious significance. (PROSE: Nobody's Gift) (PROSE: Against Nature)

Part of the first calendar was the Nemontemi, (PROSE: Against Nature) comprised of nothing days. The Aztec year had 360 days, 18 months consisting of 20 days, so the last five days of the solar year were considered "Nothing" days, days on which nothing was sold in the markets and everything stopped. (PROSE: Nobody's Gift)

Tonalpohualli

When Peri asked for the date in "human terms" when they arrived in 3174, the Sixth Doctor sarcastically informed her that it was "nine-Ehecatl day" according to the Aztecs — "but that's hardly either here or there after all that unfortunate business with Cortez". (PROSE: Burning Heart)

Other terms in the Tonalpohualli took a similar form in Nahuatl, be it Ome Ozmatli (lit. "Two Monkey"), Chicoce Cuauhtli (lit. "Six Eagle"), etc, and it was structures into groupings of thirteen days called trecenas (lit. "group of thirteen"), and a grouping of twenty trecenas. Each of days, trecenas, and solar years were given names in this form.

Nahuatl to English translations of Tonalpohualli terms

Much of the Tonalpohualli was explicitly identified, both the terms used in Nahuatl and often the English translations of the terms. (PROSE: Burning Heart) (PROSE: Against Nature)

Number in Nahuatl	Number in English		Day sign in Nahuatl	Day sign in English
Ce	One		Acatl	Reed
Ome	Two		Ocelotl	Jaguar
Yei	Three		Cuauhtli	Eagle
Nahui	Four		Cozcacuauhtli	Vulture
Macuilli	Five		Cipactli
Chicoce	Six		Ehecatl
Chicome	Seven		Calli	House
Chiconahui	Nine		Tochtli	Rabbit
Mactlactome	Twelve		Izcuintli	Dog
			Ozmatli	Monkey
			Malinalli	Grass

Religious significance

Every year, trecena, and day were presided over by specific deities in the tonalpohualli, and one could even expand this view to hold that an entire ilhuitl should be viewed in divinatory terms. The deities presiding over different parts of the tonalpohualli were as follows.

• Camaxtli
• Chantico
• Itzpapalotl
• Mixcoatl
• Quetzalcoatl
• Tepeyollotl
• Tlamatzincatl
• Tlazolteotl
• Xipe Totec
• Xochipilli

Behind the scenes

In the real world, it would be more accurate to characterise the Aztec calendar as a calendrical system rather than a single calendar, consisting of two separate calendars, the xiuhpōhualli, which measured a solar year in 365 days, and the tōnalpōhualli, which has religious, divinatory, purpose.

The xiuhpōhualli consisted of 18 twenty day "months" often referred to modernly as veintenas (lit. "group of twenty"), each of which had associated festivals, and five days added at the end of the year. While contemporary scholars agree that the Mexica were aware that the solar year was not precisely 365 days, they disagree on how the calendrical system handled this. Some have suggested that there was no correction, some that leap years were employed, Against Nature instead suggested that an additional period of 13 days was added to the end of each xiuhmolpilli (note that 13*4=52 for leap years), as is the last prominent view in the literature.

The tōnalpōhualli consisted of 20 day signs and 13 numbers used to group them into trecenas. As 13 is prime the first time the cycle of day signs and numbers will line up is 260 steps in, and there is no clear relation to solar years outside of the cycle of 52, as neither 13 nor 20 divide 365. To better explain this, consult the following chart, in English.

		...		(261 days later)	...		(366 days later)
One Reed	Eleven House		Four House	One Reed		Five Rabbit	Two Flint
Two Jaguar	Twelve Lizard		Five Lizard			Six Water
Three Eagle	Thirteen Serpent		Six Serpent			Seven Grass
Four Vulture	One Death		Seven Death			Eight Monkey
Five Motion	Two Deer		Eight Deer			Nine Grass
Six Flint	Three Rabbit		Nine Rabbit			Ten Reed
Seven Rain	Four Water		Ten Water			Eleven Jaguar
Eight Flower	Five Dog		Eleven Dog			Twelve Eagle
Nine Crocodile	Six Monkey		Twelve Monkey			Thirteen Vulture
Ten Wind	Seven Grass		Thirteen Grass			One Motion

In addition to individual days having names in the tōnalpōhualli, trecenas within the larger year were given names as well, corresponding to each of the individual day signs. Solar years were given names as well, and there is some agreement as to what those names were, but there is considerable scholarly disagreement as to how the years actually received those names, as there is in much of the scholarship related to the Aztec calendrical system.

Lawrence Burton, author of Against Nature (novel), has referenced Ross Hassig's Time, History and Belief in Aztec and Colonial Mexico in relation to some of his work, and has provided a glossary for Against Nature that includes some helpful understanding of their calendars. He has also described archaeology on similar issues as "educated guesswork", and called his own book "educated guesswork getting a bit too carried away".

For a more in depth discussion of how the in-universe calendar can be derived from the information provided in narrative sources (predominately Against Nature) rather than from real world sources, see Talk:Aztec calendar.