Year
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.
Discworld (world)
The calendar is based on a Great Year, or Astronomical Year, defined as the time it takes for the Disc 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.