The Discworld calendar was first defined in a footnote in The Colour of Magic, and has been expanded upon in later novels and the Discworld Almanack (2004). It has numurous oddities, the chief of which is its length.

The calendar is based on a Great Year, or Astronomical Year, defined as the time it takes for the Discworld to revolve once on the backs of the elephants. This lasts 800 days and contains two of each season (Midsummer occurs at a given point when the sun passes directly overhead, midwinter when it passes perpendicularly).

However most people, especially farmers, consider four seasons to be a year, so an Agricultural Year of 400 days is used for most purposes.

The agricultural year is divided into 13 months:

• Ick (16 days) (the "Dead Month")
• Offle (32 days)
• February (32 days)
• March (32 days)
• April (32 days)
• May (32 days)
• June (32 days)
• Grune (32 days)
• August (32 days)
• Spune (32 days)
• Sektober (32 days)
• Ember (32 days)
• December (32 days)

The first of Ick is Hogswatchday, the Disc's New Year, and the winter solstice from the perspective of Ankh-Morpork. In the Astronomical Year the second midwinter (the year's midway point) is called Crueltide, but to people using the Agricultural Year this is the same festival.

The calendar in general use in the Sto Plains and Ramtops ("Ankh-Morpork years") uses the agricultural year, and counts from the founding of Unseen University. Years and centuries are also given names by the UU's astrologers. 2005 AM, for instance, is the Year of the Prawn, the fifth year of the Century of the Anchovy. The majority of the Discworld novels are set in the 20th century AM, the Century of the Fruitbat, with the later ones entering the 21st, the Century of the Anchovy.

Other calendars count from various other events, and different schools of astronomy give the years different names. The Theocracy of Muntab has a calendar that counts down, rather than up. The reason for this is unknown, but people are very nervous about it reaching zero.