The Mouvelian calendar (named after Grand Patriarch Mouvel, the first high priest of the Church of Dwayna) begins counting years from the moment the gods left Tyria. This event is known as the Exodus. Years before this date are labeled BE (Before the Exodus). Years after this date are AE (After the Exodus). Years prior to the year 1 AE count down, getting smaller as they get closer to the time of the Exodus (just as they do in the Gregorian calendar).
— The Guild Wars Manuscripts
The Mouvelian calendar is the calendar used in Tyria and the most referred to calendar throughout the known world. There are 4 seasons and 360 days in the Mouvelian year. Each season is split into 90 days and corresponds to Spring, Summer, Fall, and Winter.
Between the Season of the Colossus and the Season of the Zephyr, there is a battle between the gods known as Wintersday. It is unknown if there are battles between the gods among the other seasons.
 See also