The years in this calendar are calculated from the foundation of Unseen University, which was in AM 1282. A full year in the UC is 400 days, with each season occurring once. Technically, this is half a year, due to the complicated arrangements of the sun and the resulting "double year" with each season twice. Many people seem to prefer this calendar though.
Following the Discworld people, most dates in this wiki are according to the University Calendar (UC).