The Aztecs had two calendars, one measured time called the solar year, while the other was the ritual year used to fix religious festivals.

The time-measuring calender was used to determine the best time for planting and harvesting times and to organize festivities. One year consisted of 365 days that was divided into 18 months. There were 20 day signs, (i.e. dog, water, deer, grass, etc..) and 13 day numbers. This was an continuious cycle which repeated with 5 extra days at the end of the year, which were considered to be bad-luck days when disasters were most likely to occur. A baby that was born on a bad day was not named until a good day, to change the harmful effects of the bad day.

The religious calender told when to consult the gods and lasted 260 days. Different days belonged to different gods, so days could be good or bad depending on which god's day it was. Elizabeth Hill Boone described how the religious calender worked in her book, "The Aztec World." She said, "Twenty day signs ran consecutively, from crocodile through flower, repeating after the 20th day. Beside the day signs ran 13 day numbers, 1 through 13, the numbers advancing with each day up to 13, when they repeated again with 1. Thus, the day count began with 1 Crocodile, 2 Wind, 3 House, 4 Lizard, and continued up to 13 Reed, when the numbers began again with 1: 1 Jaguar, 2 Eagle, 3 Vulture, and so forth. The 20 day signs and the 13 numbers, advancing side by side, yielded 260 uniquely named days. "

Both calenders ran at the same time and the same day in each fell at the same time every fifty-two years. So, Aztec time was divided into 52-year cycles. In Aztec religion, the destruction of every era always occurred on the last day of each 52 year cycle (although each era lasted for several of these cycles. This was the time when the gods could decide to destroy humanity, so the most important religious event in Aztec life happened. This was called the New Fire Ceremony and was held for five days before the end of the cycle. The fires of all religious alter were put out and every household destroyed all furniture and belonging and mourned for the world. On the fnal day the priests to a crater in the Valley of Mexico called the Hill of the Star, and waited to see if the constellation of Pleiades would appear. If is appeared, the world would continue for another 52 year cycle. A fire would then be lit by the priests in the body of an animal carcass, and all of the fires in the valley would be lit from this new fire. The next day many ceremonies with sarifices and blood-letting would happen. The people had feasts, and rebuilt furniture and regained possessions.

At the center of the Aztecs' calendar stone is the sun god. He is surrounded by symbols of the five world creations. The symbols of the 20 days of the solar month are depicted on the stone. Also, eclipses of the sun were predicted by the calendar stone.