Aztec calendar

The Aztec calendar was a human dating system to which the Sixth Doctor sarcastically referred when Peri asked for the date in "human terms" when they arrived in 3174. He informed her that the day 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)

A Nothing day was a part of the Aztec calendar. The Aztec year had 360 days, so the last five days of the year were considered "Nothing" days, days on which nothing was sold in the markets and everything stopped. (PROSE: Nobody's Gift)