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. Note here that no relationship was given generally between symbol and number aside from when the two were used in a date together. This is merely a table that includes both. (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 , which measured a solar year in 365 days, and the , 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 workIt's Been a While, and has provided a glossary for Against Nature that includes some helpful understanding of their calendars.Against Nature Nahuatl Glossary 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.