0 (year)

Year Zero also refers to the Cambodian genocide of 1975–79.

There is no year 0 in either the Gregorian calendar or the Julian calendar. The year 1 BC immediately precedes AD 1. Historians adopted this convention after it was first used by Bede in his Historia Ecclesiastica Gentis Anglorum (Ecclesiastical History of the English People, 731). In book 1 compare chapters 1 and 3. He did not use a year zero because the counting numbers used to number years begin with one, not zero. Nevertheless, he was well aware of zero as a number because zero was the first epact of the nineteen-year cycle used to calculate the date of Easter as he explained in his De Temporum Ratione (On the Reckoning of Time, 725). The Latin word nullae meaning "nothing" was used for this zero epact, whereas all other epacts were numbered via Roman numerals.

Confusion over year 0 versus 1, and a preference for anniversaries in "round" figures, led to the celebration of the new millennium both in 2000 and 2001.

Year 0 is used in astronomical year numbering and is equivalent to 1 BC in the Gregorian or Julian calendars. Its first use is traditionally attributed to Jacques Cassini in his Tables astronomiques (Astronomical Tables, 1740) wherein he explained his reasons for doing so. But Phillipe de La Hire had used it earlier in 1702 in his Tabulæ Astronomicæ (Astronomical Tables) in the form Christum o. (Christ 0), without explanation.

All eras used with Hindu calendars, such as the Saka era, begin with a year zero because all of these calendars use elapsed, expired, or complete years, in contrast with most other calendars which use current years. A complete year had not yet elapsed for any date in the year beginning at the epoch, thus that could not be year one—instead, it was year zero.