I have discovered a couple of problems with the way the Aztec Astrologer calculates the Aztec date for a given day. You should be aware of these if you are basing your life decisions on these pages (which I hope nobody is doing!).

Firstly, the Aztec calendar system is based on a 52-year time period after which both the solar and sacred calendars come into line, with 365 days in each year. Without the addition of leap days, as in the Gregorian calendar, this system will get hopelessly out of phase with the sun. Vaillant (1965) and others indicate that the Aztecs were well aware of this, and possibly added extra days to the 5-day "Nemontemi" at the end of each year, or perhaps included a special feast every eight years. The calendar system given by Tunnicliffe (1979) includes the rougher solution of adding 13 nameless days to the end of each 52-year bundle, bringing the calendar more in line with the sun (as much as the old Julian calendar was).

However, I noticed the other day that I had omitted this 13-day period in my algorithm. This is not too devastating - none of the other Aztec calendars I have seen on the web include any correction for the leap years.

More importantly, the calendar system given by Tunnicliffe (1979) is supposed to agree with the day 13 Aug 1521 being the Aztec day 1 Coatl in the year 3 Calli. But even with the addition of the 13-day correction, I cannot make the system match the Gregorian day 23 Aug 1521 (which was 13 Aug 1521 in the Julian calendar) to that Aztec day. This is a bit worrying, meaning that the dates (and so also the advice) given is wrong, hence this warning.

I won't change it until I've done some more research to find out the most authentic way of handling the leap year correction, which I think is more important in the long run, so until then please don't blindly follow what the Aztec Astrologer tells you!

References

Tunnicliffe, K.C. (1979) Aztec Astrology, Fowler.

Vaillant, G.C. (1965) Aztecs of Mexico, Penguin Books.