Azteca/Mexica Calendar Correlations: the Good, the Bad, and the Completely Useless

There are many Aztec/Mexica calendar correlations out there and it can be very intimidating to try to figure out which correlation is the most accurate.  This entry will help sort out the different calendar correlations in a simplified way.  Please note that this is by no means an exhaustive list of all of the calendar correlations available but rather the most widely used by scholars and cultural practitioners.  Hopefully, by understanding how the Azteca/Mexica calendar works and correlates to Earth’s position around the sun, you will be able to critique other calendar correlations that you come upon.

Only recently has there been a good calendar correlation that actually aligns to the calendar that was maintained in Tenochtitlan at the time of the arrival of the Spaniards.  The fact that it has taken so long for us to have an accurate calendar correlation available to us is interesting considering that we have a wide range of primary sources that reference the calendar.  The problem we have when it comes to primary sources is the Spanish and Native chroniclers who wrote about the calendar pretty much all disagreed with each other.  This point illustrates how bad the problem is: in 1967, Alfonso Caso undertook the most exhaustive study on the Nahua calendar surveying 42 sources ranging from the 16th to the 20th century. Caso documented the starting months of the year in these sources and according to his analysis, 14 cited Tlacaxipehualiztli, 14 cited Atlcahualo, 7 cited Izcalli, 3 cited Tititl, 2 cited Atemoztli, 1 cited Panquetzalli, and 1 cited Toxcatl.  It doesn’t end there and it is a similar situation when one wants to find out about the first day of the year and intercalary corrections.  At one point in the Florentine Codex, two Spanish chroniclers actually have a heated argument about whether or not the Native Mexican calendars incorporated a leap year.  The situation can be explained with this quote from Zelia Nuttal:


Therefore, the reason for their disagreements is probably related to the fact that the Native and Spanish chroniclers who were documenting the calendar were working years after the Spanish conquest, long after the calendar fell into disrepair as the tonalpoque were no longer making the necessary corrections.   Fortunately for us however, there are 13 surviving Pre-Cuauhtemoc books, many of which contain valuable information regarding the calendar.  In addition, our ancestors were expert astronomers and much of their work is still available to us in the form of ceremonial buildings and inscriptions.  As you can imagine, the most reliable calendar correlations are those that align to these valuable pre-cuauhtemoc sources.  The primary sources dealing with the calendar are not completely useless though because when we compare the pre-cuauhtemoc sources to the works of the Spanish and Native chroniclers it is clear that the following historians had a good understanding of the workings of the calendar: Mariano Veytia, Juan Tovar, Diego Duran, Toribio Motolinia, and Ixtlilxochitl.

Solar Calendars 101

To understand the Azteca/Mexica calendar, it is important that you first understand how solar calendars work.  Put simply, solar calendars use dates to indicate the position of Earth on its revolution around the sun.  Solar calendars can be useful for helping people understand when the optimal time to plant their crops is or when the seasons change for example.  The problem that all solar calendars face is the Earth takes 365.24255 days to orbit the sun which can’t be expressed with whole numbers.  Solar calendars get around this problem by adding an extra day to their calendars every four years (a leap year).  What the leap year does is adds .25 days to the solar calendar each year which makes each year 365.25 days long to better approximate the actual solar year of 365.24255.

The leap year does not address the problem fully however because it actually adds .01 extra days every year.  This results in an extra day added to the calendar about every 100 years which throws the calendar off because it results in the days taking place 1 day later than they are supposed to.  The Gregorian calendar is the calendar used in most places of the world and it has a correction for that error built in so that the extra day is not added every 100 years (previously, the Julian calendar was used which incorporated a leap year but no secondary correction).  You can read more about that correction elsewhere but for our purposes, that explanation is sufficient.  

Native Mexican calendars

Native Mexicans utilized a solar calendar, called the xiuhpohualli in combination with a ceremonial calendar, called the tonalpohualli.  The xiuhpohualli functions in the same way that our own (gregorian) calendar works.  To compare, our calendar contains 12 months with either 28, 29, 30, 31 days which add up to 365 days while the xiuhpohualli contains 18 months with 20 days each and a 19th month with 5 days which add up to 365 days as well.  To specify a day in the gregorian calendar, we say the first day of January.  To specify a day in the xiuhpohualli calendar, we say the first day of Tlacaxipehualiztli.  Because they are both solar calendars, they are used to designate a particular time of the year that is recurrent.  We know for example that December will always be cold because it falls in winter.  We can say the same for the month of Panquetzaliztli in the Xiuhpohualli because it also occurs during winter every year.

Click here to learn more about the months of the Xiuhpohualli and here to learn more about how the years are named.

The tonalpohualli on the other hand is a ceremonial calendar which runs along side the xiuhpohualli.  It only has 260 days so it can’t be used as a solar calendar and is therefore useless for telling us what time of year it is when used alone.  We can’t for instance say that the day 12-Xochitl will always fall during winter because it will take place during a different part of the xiuhpohualli every year.  One year it might fall on 13-Tlacaxipehualiztli whereas the following year it might fall on 13-Panquetzaliztli.  In fact, in some cases, 12-Xochitl appears twice in the same year.  In Pre-Cuauhtemoc society however, the tonalpohualli was very important because this is how people got their calendar names.  It was also used to determine how strong or weak the tonalli of the person was which was then used to determine fate.  A person born on the day 13-Ozomahtli for example was said to be highly respected because he was compassionate, and was a great speaker whereas a person born on the day 6-Cuetzpallin would have a life full of misery because he was very hot-tempered and he talked too much.  

Although the tonalpohualli contains only 260 days, when it is combined with the xiuhpohualli, the tonalpohualli count falls into a recurring and predictable pattern which led Native people and also modern day researchers to treat it as another set of solar count days.  In pre-cuauhtemoc times, people would have grown accustomed to having cipactli days start each month for the first 18 months and having xochitl ending them in acatl years for example.  In addition, in calli years, ozomahtli falls on the first day of each month while itzquintli  falls on the last day of each month.  Tonalpohualli days are also grouped into 20 13-day periods called trecenas which further assisted people in understanding where the tonalpohualli count was in relation to the solar calendar.  In fact, when we reconstruct the original calendar correlation we find that Native and Spanish chroniclers were much more accurate with their documentation of the tonalpohualli days compared to their documentation of the days of the months.

Click here to learn about the days of the Tonalpohualli and here to learn more about how the trecenas are used to organize the Tonalpohualli.

The Calendar Correlations

Zelia Nuttal (September 6, 1857 – April 12, 1933) – Although her work has been largely ignored, Zelia Nuttal made some major discoveries which have contributed to our understanding of the Azteca/Mexica calendar.

– She was the first to identify the pattern of four starting days that match up with the year bearer:  tochtli years always start on cozcacuauhtli days, Acatl years always start on cipactli days, Tecpatl years always start on miquiztli days, Kalli years always start on ozomatli days
– She discovered that the first day of the year was linked to the spring equinox
– She was the first to realize the connection between the Native Mexican practice of tracking the sun and the calendar (this seems odd considering the entire purpose of a solar calendar is to track the position of the Earth in relation to the sun)
– She was the first to correlate the start of the new year to the first day of Tlacaxipehualiztli

With the above discoveries, it should have been the end of the discussion however, for reasons unknown, she did not take the next step and apply the necessary intercalary corrections (such as the leap year) to create a functional calendar.

To read more about Nuttal’s calendar correlation, click here: Note_on_the_Ancient_Mexican_Calendar_Sys

After Zelia Nuttal, the next researcher to contribute to the calendar correlation was Alfonso Caso (February 1, 1896 in Mexico City – November 30, 1970 in Mexico City) which is the calendar correlation that can be found at  The Caso count is also the one used by U.S. scholars specializing in Mesoamerican history.  For reasons unknown, Caso seemed to have started from scratch and did not consult Zelia Nuttal’s work. His major contribution was the discovery of three correlation dates that match up when we count backward from 1-coatl.  1-Coatl corresponds to 8-13-1521, the Fall of Tenochtitlan and it is the one date that is consistently documented by many different people, both Native and Spanish, which greatly increases it’s validity.  It’s important to note that without dates that anchor the Native calendar to a specific time of year, we would not be able to reconstruct the calendar at all because there would be no starting point in which to reconstruct the calendar.  Here are his three correlation dates:

08/11/1519 2- Quecholli (Xiuhpohualli) 8-Ehecatl (Tonalpohualli) Arrival of Spaniards to Tenochtitlan 
30/06/1520 11-Tecuilhuitontli (Xiuhpohualli), 8-Cozcacuauhtli (Tonalpohualli) Noche Triste 
01/07/1520 12-Tecuilhuitontli (Xiuhpoalli), 9-Ollin (Tonalpohualli) Noche Triste
13/08/1521 15-Miccailhuitontli (Xiuhpohualli) 1-Coatl (Tonalpohualli) Fall of Tenochtitlan 

For reasons unknown, Caso did not incorporate a leap year which has resulted in his calendar moving forward 120 days.  Days in Caso’s count for example that fall in spring should actually be taking place in winter.

I recently asked Anthony Aveni, widely considered to be the founding father of Mesoamerican astroarchaeology, about why U.S. scholars insist on using Caso’ calendar correlation knowing that it is not accurate.  This was my question:

and this was his response:

Translation: scholars know that the calendar is not accurate however they all do not feel the need to correct it.

This is an extraordinary response from the authority in the field.  I believe he and other scholars have based so much of their work on the Caso correlation that conceding it is inaccurate would result in the nullification of much of their work.  This is something they are clearly not willing to do.

To read more about Alfonso Caso’s calendar correlation, click here: CasoLosCalendariosPrehispanicos

Click here to see the tonalpohualli and xiuhpohualli dates from 1519-1521 (remember that Caso did not use leap years but a leap year has been applied in the acatl year to show the successful correlation between the three Caso dates):

Rafael Tena’s calendar correlation builds on the work of Alfonso Caso with the addition of a leap year although he also appears to have ignored the work of Zelia Nuttal.  Tena’s calendar correlation is the one that is widely used by Mexican scholars specializing in Mesoamerican history.  Because he ignores Nuttal’s work, Tena relies on Spanish chroniclers to determine the starting point of his calendar which as we discussed above is a big mistake.  Tena starts his years on February 26th on the day 1 Atlcahualo which leaves him way off the mark of the  correct correlation.  Because he began counting from 1-Coatl and incorporated leap years however, his tonalpohualli count is only off by 7 days.    His tonalpohualli count is off because although he incorporated a leap year, he failed to incorporate a secondary correction for the extra day that accumulates.

Arturo Meza – the Arturo Meza calendar correlation  is widely used by danzantes and other cultural practitioners in Mexico and the U.S.  This calendar correlation is also utilized by many Mexicayotl teachers including Mazatzin Aztekayolokalli, and Ocelocoatl.  Arturo Meza’s calendar correlation is almost entirely based on Spanish sources so it has the same problems as the Rafael Tena calendar: incorrect starting day and incorrect month order.  Additionally, Meza does not run the tonalpohualli count through a full 365 days; he runs the tonalpohualli through 360 days, stops, interjects a separate nemontemi count, and then starts the new year again on the same day every year: Cipactli.

Curiously, Meza incorporates a leap year by adding 1/4 a day every year by starting the day of each consecutive year at different times: midnight, sunrise, noon, and sunset.  These practices are not supported by any primary source document.  He also incorporates a secondary intercalary correction that he says derives from oral history.  This calendar correlation  is unsupported by any pre-cuauhtemoc or Native Mexican astronomical sources and it does not build on the work of any of the above researchers.  As a result of the aforementioned errors, Meza’s correlation is by far the most inaccurate available.

Ruben Ochoa – In contrast to Tena and Caso, Ruben Ochoa’s calendar correlation utilizes Zelia Nuttal’s discoveries and incorporates appropriate intercalary corrections to ensure the days remain anchored to Earth’s position to the sun without accumulated error.  In addition to a leap year, his correlation also utilizes ancient Mexican astronomical markers to prevent the accumulation of an extra day every 100 years.  Ruben Ochoa’s calendar correlation is also the only one that relies entirely on Pre-Cuauhtemoc sources and Native Mexican astronomy which avoids any reliance on Spanish primary sources which we have demonstrated are all contradictory.

Here are the major discoveries that Ruben Ochoa made which have contributed to our understanding of the Azteca/Mexica calendar.

– He confirmed the validity of the three Caso dates (see above) by adding a leap year to the year 1-Acatl (1519)
– He was the first to confirm Nuttal’s pattern of four starting days that match up with the year bearer ( tochtli years always start on cozcacuauhtli days, Acatl years always start on cipactli days, Tecpatl years always start on miquiztli days, Kalli years always start on ozomatli days) through the correlation of Pre-Cuauhtemoc books including the codex borgia, laud, and Ferjervary-Mayer.
– He confirmed that the first day of the year is linked to the spring equinox by correlating Native astronomical markers to the calendar.

If you are interested in learning more about the sources used to support the Ruben Ochoa calendar correlation, please visit:

Click here to view a video on the Pre-Columbian sources discovered by Ruben Ochoa:

To see the Ruben Ochoa calendar count which eventually will contain dates going back to 1519, please visit: 

Here’s a description of how the calendar dates from the website above are organized:


As you can see, the Ruben Ochoa calendar correlation is the only one available that incorporates all of the components required of a solar calendar to guarantee it remains accurate.  His correlation successfully builds off of the work of his predecessors to generate both an accurate xiuhpohualli count and an accurate tonalpohualli.

The Secondary Correction (read if you want to know more about how the astronomical markers work as the secondary correction for the Xiuhpohualli)

In addition to over-reliance on contradictory primary sources, many people in the past have tried to correlate the Azteca/Mexica calendar with the Gregorian calendar. The problem with this approach is that it makes us dependent on the Gregorian calendar and it throws off the Azteca/Mexica calendar. This is because the Gregorian calendar does not utilize astronomical observations such as equinoxes and solstices to anchor itself to the correct position of the sun. It utilizes the leap year and a mathematical secondary correction that are independent of any astronomical event.  Tena’s correlation for example starts on February 26th and Meza’s correlation starts on March 11th but those two dates do not correlate to any observable astronomical phenomena which forces the use of the Gregorian calendar as a reference point because it is not possible to know it is the first day of the new year without first knowing the Gregorian date.  The Azteca/Mexica calendar on the other hand utilizes both the astronomical observations of the spring equinox and winter solstice as anchor points guaranteeing that the calendar never strays from the correct position of the sun.

Here in this graphic from the Codex Borgia, we can see clearly the two spring equinox and winter solstice anchor points. We can also see that each graphic contains correlation days for the year 1-Acatl. The day 1-Cipactli corresponds to exactly one day after the observable spring equinox in Mexico. The day 4-Ollin corresponds to exactly one day after the observable winter solstice in Mexico. Astronomical markers all across Mexico track the spring equinox and winter solstice and were used to keep the calendar aligned with the sun. Anthony Aveni has identified at least two sighting devices from the Bodley and Selden that the ancients used to make these precise observations. None of the most widely used calendar correlations (Arturo Meza, Alfonso Caso, and Rafael Tena) align with these two crucial correlation pages. The Ruben Ochoa calendar correlation is the only one that aligns with these crucial correlation pages and is also the only one that utilizes the same astronomical practices as the Azteca/Mexica to keep the calendar aligned with the path of the sun without relying on the Gregorian calendar at all.

blog post

*Post script*

After I published this article, somebody argued that all of the above calendar correlations can be correct and there is not only one that is correct.  I thought it was important to point out this article is referring to the calendar that was used in Mexico-Tenochtitlan at the time of the conquest.  Arguing that there are many correct calendar correlations in this context is equivalent to me saying today is January 19th and somebody else saying today is June 19th and us both being correct.  Of course this is ridiculous.  If we both claim to be using the Gregorian calendar, only one of us can be correct.  This is the same situation that we find ourselves in with all of the different correlations that are available to us.  Here is an image I made that illustrates the accuracy of each calendar correlation mentioned in this article:


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    1. This is amazing information. I appreciate it and look forward to purchase your planner next year.
      Too bad most people today are following the Alfonso Caso correlation, including mayas and nahuas alike. I guess they now prefer to be all united using the same mistaken calendar than to break communion following a correct calendar.
      I follow the website in which they intelligently criticize all fraudulent calendars that have been claimed as “mayan” so far. They also claim to have the perfect Mayan calendar inherited by tradition through maya wise old people. However, I noticed that it is aligned with the Aztec Caso-Nicholson’s correlation calendar, unfortunately.


  1. Thanks for the analysis bro. Good info to consider.
    I haven’t done it for a while but for some years I followed the Meza count and took notes of daily themes, occurrences, frequencies and such within my life, to which it correlated very interestingly. You’ve inspired me to revisit the Tonalpohualli and give the Ochoa count a spin. Tlazocamati.


  2. In Ruben Ochoa’s correlation, is the sixth day added to the nemontemi not counted in the tonalpohualli? If a sixth nemontemi day is added, wouldn’t this throw off the year-naming scheme? This would seem to be a problem for any correlation that accounts for the solar year not being exactly 365 days.


    1. The naming scheme is always tied to the spring equinox. adding a sixth nemontemi day only becomes necessary when the calendar has trailed off from the correct spot in the solar year. Therefore, the answer to your question is no since the naming scheme is always linked to the spring equinox.


  3. Today, 6th may 2018 it is showing “12 ehecatl” in the tonalpohualli link, and i guess it should be 13 ehecatl using the Ruben Uchoa correlation, for yesterday 5th may 2018 it was ok as it shows 12 TziPactli and for tomorrow 7th may 2018 it is ok too as it shows 1 calli. Apologize for posting it here but i could not find anywhere else a link for an “admin” or contact email. Feel free to delete this post and thaks very much evebody involved in keeping Sincerly Don from Brasil.


  4. totally drowning. I see no 1-Reed in 1519 which starts on 1 Ahau Mayan or 1-Xochitl. The day 1-Reed (1-Ben) falls in 1491AD for Pop new year. How can you call a year 1-Reed if the new year doesn’t fall on the day 1-Reed.


    1. The year bearer is not the same as the first day of the year. The year bearer is the result of running numbers 1-13 and 4 signs (tochtli, calli, acatl, tecpatl) in the Xiuhpohualli cycle which repeats every 52 years.


  5. This is fantastic and extremely helpful. THANK YOU. I studied Náhuatl in a village in Guerrero and, unfortunately, the calendar has been lost there (at least from what I’ve experienced). I started keeping the calendar when I was about 13, but became frustrated by discrepancies across resources available to me growing up in the suburbs of Chicago. I’m am going to begin keeping it again, and feel very fortunate to have found your site. Tlaxtlawi


    1. niltze Paul,

      Tlazokamati for the feedback! I too was frustrated for many years by the discrepancies in the primary sources. Everyone one I’d ask about it were so confident that their correlation was the most accurate but nobody could actually explain why. I did tons of research over the years and was very fortunate to meet Ruben Ochoa because he literally taught me how to read the pre-columbian codices and identify the evidence. The amount of evidence out there is staggering but most just don’t know how to read it. I think I’ll put up some posts that go over some of the evidence for those who interested.


  6. Hey,

    I wasn’t sure where to contact the site, but I think the calendar app has a small error. As of now, it is adding the 6th nemontemi day on the first day after the spring equinox (March 21st, 2019) of the current year of (6-xochitl, chicuase tochtli xihuitl) instead of 7-Cipactli, 1-Tlacaxipe-,7-Acatl xihuitl. From reading the article on the Ochoa correlation, I thought the 6th nemontemi occured in acatl years?

    Thanks for all your work. I really love the site.


    1. Daniel,

      Great catch! – It shows you understand our calendar very well. The 21st is the first day after the spring equinox only if you go by the hyper technical definition of the equinox, which this year occurs just before 3p on the 20th PST, but the first observed equinox sunrise would actually be in the 21st, making the 22nd the first day after the observed equinox sunrise. Since we are tying our correlation directly to the equinox just as our ancestors did, we must remember that the spring equinox would not be observed until sunrise on the 21st at the Templo Mayor (or any other Native astronomical marker). It turns out that this technique of tying our calendar to the spring equinox generates a natural pattern in which the leap year falls in different years, not only Akatl years. The next leap day will be added in another tochtli year, 2022, and then will shift back to akatl for a bit. As long as our beginning tonalli occurs a day after the observable spring equinox, we know we are calibrated. Where are you located? If you are in a L.A., you can schedule to meet with Ruben so he can explain in more detail.


      1. I’m originally from L.A., but am currently living in San Antonio, Tx. I would love a chance to talk with Mr. Ochoa and thank him for the work he’s done (and yourselves as well). I have tried to better understand the workings of the calendar myself and ran into some of the same issues that this correlation addresses. Feel free to reach me by email for additional contact info. I appreciate the explanation and the dedication to preserving and promoting the rich culture of our people.

        Tlasokamati Huel Miak,


  7. Greetings,
    I have a question about the Ruben Ochoa’s correlation because, I think, it could be a mistake on the leap year of 1519 1-Coatl or in how does the mexica priests measure the vernal equinox, or in the NASA calcules.
    I’ve been searching in the NASA for the dates and time of the equinox at 1500’s and in 1519 the vernal equinox ocurred at March 21 at 16:05 (in the Proleptic Gregorian Calendar) so… I understood in the answer above, that they corrected their calendar one day after they’ve seen (measured) the vernal equinox. In this case: In 1519 I guess, they noticed the equinox at March 22 (at noon) and started the Xipehualiztli at March 23 but in the correlation, Ochoa starts one day before, could they measure the equinox not at noon? Maybe the date of the Arrival of the Spaniards is wrong? Please, answer me I really like all the work in this project and this question cause me anxiety! Thanks a lot!


    1. Hi Leonardo,

      Counting backwards from 1-Coatl, the first day of 1521 occurs on March 12, 1521. The spring equinox according to NASA occurred on Mar 10, 1521 10:45 pm LMT. Since the sun was not visible, our ancestors had no way of knowing the spring equinox was occurring at that time and as a result the spring equinox was not visible until the morning of March 11th. Then they started the new year on the following day, March 12, 1521.

      If you continue counting backwards from there, the first day of 1520 occurs on March 12, 1520. The spring equinox occurred on Mar 10, 1520 5:01 pm LMT. Again, since the spring equinox occurred at night, it would not have been visible until the morning of March 11. Which is why the new year started on the following day March 12, 1520.

      If you continue counting backwards from there, you will notice that the gregorian date for the Mexica New Year actually SHIFTS forward one day to 13/03/1519. However the spring equinox occurred on Mar 11 at 11:09 am LMT that year. This is why the leap year must be applied at the end of 1519. Without it, the 1519 Mexica new year shifts forward. To answer you question, yes, it does appear that the time around 11:09 am LMT is the cut – off time. If you look at the starting days of all the years and compare it to the spring equinoxes you will find a definitive cut-off time. I don’t think I have seen any observable spring equinoxes later than noon.


    1. You should always use the time zone in Mexico since the calendar is tied to the passing of time there and nowhere else.


  8. Hello,

    I’m a bit confused as to some of the details of the Mesoamerican calendar, in particular how Caso, Tena, Meza, and Ochoa made their correlations.

    Firstly, I tried making the count myself by printing out blank calendar squares and writing in the tonalli from the begining of 1519 to the end of 1521 of the Gregorian calendar using the correlation dates of the Julian calendar given in the article. I used and online converter to find the Gregorian dates for the Arrival of the Spaniards to Tenochtitlan (8/11/1519, Julian; 8/21/1519. Gregorian); La Noche Triste (6/30-7/1/1520 Julian; 7/10-7/11/1520 Gregorian); and The Fall of Tenochtitlan (8/13/1521, Julian; 8/23/1521, Gregorian). I started the counting from the Arrival of the Spaniards in the day 8-Ehecatl to the Noche Triste to see if it mathched up, while using the link to the Ochoa count for these dates, thinking the link uses the Julian Calendar.

    I noticed however that the date for the arrival of the Spaniards in the link states the day was 10-Acatl and not 8-Ehecatl. Did I make a mistake in the conversion or is this a mistake from the link?

    Also, if I wanted to start over and do the count myself, how would I go about it? How do I begin? and how will I know if it is the correct count? With all these correlations, even ones not mentioned in the article, how do you know which one is the right one?


    1. Armando,

      To do the count yourself you need two things: 1) An Indigenous anchor date and 2) correlation of the calendar to a day during the year. While everyone who has attempted to reconfigure the calendar has used an anchor date (1-Coatl), only Ruben Ochoa has correlated the calendar to a day during year. Therefore, as I state in the blogpost, Ruben Ochoa’s correlation is the most accurate of them all. Start with 13/08/1521 1-Coatl Fall of Tenochtitlan then count backwards. You will then find that the day 3-Ozomatli lands exactly one day after the observable spring equinox in Mexico. This is the formula you will use as you continue to reconstruct the calendar. If the incorrect day (there are only four possible signs: ozomahtli, cipactli, miquiztli, and cozcacuauhtli). for any given year lands on the day after the spring equinox, you made an error which is most likely applying the leap day in the wrong year. You can find 1519-1521 mapped out here as an example:


      1. Hello,

        Thank you for your response. I will attempt to try and do the count myself again!

        Just one more question, are the dates used on the link provided from the Gregorian calendar or the Julian calendar? I read somewhere that most historians use the Julian calendar for dates after the 1560’s or 1580’s if I remember correctly. I just wanted to be sure of that.




        1. Any date you find from that time period is Julian because the Gregorian did not begin until 15 October 1582.


  9. Hello,
    Is the website which uses Caso’s interpretation fairly “correct”?


    1. No,its not fairly correct. Every 4 years it trails away further because there is no leap year correction. Of all the counts it is 2nd to last in accuracy.


      1. Which is the best site to convert dates? It is possible to have recent dates translated? Dec 16, 1982 and Aug 31, 2020.


  10. Hello,

    Searching for more correlations, I’ve found one very close to Ochoa and has made for Carmen Aguilera, based on Motolinia’s writings exposes some interesting events: First, the Huey Teocalli is oriented to notice the equinoxes and solstices with the double chapel at the top, in fact the spring equinox matched in the middle of them.

    Second, and here is in disagreement with Ochoa’s, is that the all the hollidays feasts of each month falls on the day 20 of the month except for Spring Equinox; So, Summer Solstice falls on 20 – Etzalcualiztli, Winter Solstice on 20 – Panquetzaliztli and Spring Equinox on 7 – Tlacaxipehualiztli; in Ochoa’s for the 2020 year, the Summer Solstice falls on 12 – Etzalcualiztli, Winter Solstice on 16 – Panquetzaliztli and Spring Equinox on 1 – Tlacaxipehualiztli.

    In fact, Aguilera set her calendar with all the dates provided for Caso and added the openning of the Huey Teocalli chaired for Ahuizotl at the day 20 – Panquetzaliztli (7 – Acatl) of the year 8 – Acatl (1487), this date is sculpted on the Huey Teocalli and in the Codex Telleriano – Remensis.
    Aguilera supports her correlation with another astronomical events: the first cenital passage, falling at the beginnig of (2-)Toxcatl, and the second passage falling at 20-Hueytecuilhuitl.

    And here I’ve got several questions: Does the date of the Huey Teocalli matches in the reverse Ochoa’s count? Could the mesoamerican calendar be missmatched the beginning few days to the spring equinox? or probably the calendar starts at Atlcahualo?

    I share the info
    1) Aguilera correlation
    2) Date of Ahuizotl’s openning of the Huey Teocalli

    Bests Regards


    1. Hi Leonardo,

      Thank you very much for sharing your sources for Aguilera’s correlation – Its always a good thing to research as many sources as you can. Just for some context: The basis of Aguilera’s correlation is also used by Rafael Tena. Tena and Aguilera both agree that Caso was wrong in asserting there was no leap year applied to the Mexica calendar. As a result, if you look at her tables she has in her article for the years 1487 and 1521, you will see the tonalpohualli portion is exactly the same as the Ruben Ochoa correlation. The problem with Aguilera and Tena is that they do not apply a secondary correction for the day that accumulates every 100+ years. Therefore if you look at Tena’s calendar today you will see it has trailed off approximately 3 days from the correct date. Regarding the months, there is nothing in the stone inscription of Ahuitzotls opening of the Huey Teocalli that suggests it occurred on or near the winter solstice as Aguilera claims. Her only evidence she provides is a Caso citation but she herself said Caso’s correlation was incorrect so why she would use him as evidence doesn’t make much sense. Her other major claim that you cited that Summer Solstice falls on 20 – Etzalcualiztli, and 20 – Panquetzaliztli on the winter solstice is not mathematically possible (This is because the Earth’s orbit forms an elliptical pattern, not a circular pattern). This is an error she made. You can run the calendar from 20-Etzalcualiztli to 20 – Panquetzaliztli yourself to see that it is not possible for both solstices to land precisely on the 20th of those months. Also, some sources say celebrations took place on the 20th day, others in the middle of the month, and still others in the beginning of the month – there is no consensus on the issue at all so she is cherry-picking sources that fit her claim and ignoring the others that contradict it. This is why it is very important to rely only on Pre-Columbian sources when correlating the calendar.


    2. To answer you questions directly:

      in the Ruben Ochoa correlation the day 7 acatl, year 8 acatl falls on 13 Tlacaxipehualiztlii which is located in Tonalko and thus matches the image from the Telleriano Remensis if we assume no error occurred as Aguilera claims. An inauguration near the spring equinox makes sense considering this is the new year in our calendar.

      If the calendar started with Atlcahualo, it would throw everything off and the seasons would not match the appropriate months anymore. The winter solstice for example would not occur during its proper month, Panketzaliztli and the spring equinox would not occur during its proper month, Tlacaxipehualiiztli.


  11. Ruben,

    Will you please provide a source claiming the entry into Tenochtitlán took place on the second day of the month of Quecholli? I believe some correlations place the entry on the ninth day of the same month.

    Thanks in advance,



    1. Erik,

      There are no secondary sources that correctly correlate the days of the month exactly if we follow the premise that the new year begins on the first day of Tlacaxipehualiztli but as you mentioned there are several that are very close.

      Page 33 of the Codex Borgia is the beginning of a sequence that depicts the months of the year. This is the only Pre-Columbian depiction of the months that exists so it is extremely important. This page specifies that Tlacaxipehualiztli is the first month of the year. Hence, if you count from the first day of Tlacaxipehualiztli on the first day of the month, you will find that 2-Quecholli occurs precisely on the day 8-Ehecatl which is November 8, 1519 in the Spanish version of the calendar.


  12. Studying the calendars for aprox 20 years I feel Caso’s correlation is the most correct. If that is we take into account that the Western calendar changed after the julian/Gregorian switch (in 1582 after the Conquest, thus after the anchor dates) and that leap years were taken into account in a yet undiscovered way. I believe it was the feast of Huauhquiltamalcualiztli that accomplished this, after all, it was celebrated every 4 years. The beginning of the calendar in my opinion was beginning of February. Put math and anchor dates aside and look at HOW the months were celebrated and to which deities. That should be our biggest clue. If in your correlation harvest or planting months happen in the middle of summer and middle of winter YOUR CORRELATION IS WRONG, let’s stop over thinking this


    1. Ok Edy,

      Caso’s correlation has Tlacaxipehualiztli starting in the middle of November. Tlacaxipehualiztli is a month firmly connected with the Spring equinox. Using your own criteria, we can see Caso was wayyyyy off the mark.


  13. Maybe there is another evidence of the correlation of the two calendars.

    On the pages 49-52 of the Codex Borgia are on the left side in the upper corner (of the current cell) animated day signs with 4 day disks each time.

    Under these signs are a little bit on the right side year signs with 4 disks, too.

    On page 49 e.g. day 4 ozomatli and year 4 calli.

    Maybe these indicates not just the beginning of a new year, but also a special feast (the four quarters of the 52-year-period?).

    Another evidence could be the fact, that every year begins with a day number one „disk“ higher than the previous year.

    It is almost impossible to ignore all these evidences – Ruben‘s count must be the correct one.

    Greetings, Te


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