The number of full days that is treated as one year for civil or religious purposes. This number must vary from year to year in any calendar to keep it in step with astronomical cycles such as the orbital periods of the Earth or Moon, since a fraction of a day is involved in each case. Years are counted from some epoch. In the Gregorian calendar this epoch is the birth of Christ, estimated (probably incorrectly) to have occurred in ad 1. Years before this are designated bc, but there was no calendar year zero. For astronomical computing purposes, the calendar year 1 bc is designated 0, 2 bc is −1, and so on.