- 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). 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 nulla meaning "nothing" was used for this zero epact, whereas all other epacts were numbered via Roman numerals.
Bede's non-use of year zero can be deduced from book I of his Ecclesiastical History. In chapter II Bede stated that Julius Caesar invaded Britain "in the year 693 after the building of Rome, but the sixtieth year before the incarnation of our Lord", while stating in chapter III, "in the year of Rome 798, Claudius" also invaded Britain and "within a very few days" … "concluded the war in" … "the fortysixth [year] from the incarnation of our Lord". Although both dates are wrong, they are sufficient to conclude that Bede did not include a year zero between BC and AD: 798 − 693 + 1 (because the years are inclusive) = 106, but 60 + 46 = 106, which leaves no room for a year zero. This is also the first use of 'BC', which continued to be used sporadically throughout the Middle Ages (usually by repeating this singular use of Bede's, albeit with a correct year) until its first extensive use in Fasciculus Temporum by Werner Rolevinck in 1474, along with years of the world.
Year 0 is used in astronomical year numbering and is equivalent to 1 BC in the Gregorian or Julian calendars. All years before 0 are negative numbers, thus −1 = 2 BC. The first use of an astronomical year zero 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 Tabulę Astronomicę (Astronomical Tables) in the form Christum o. (Christ 0), without explanation. However, neither Cassini nor La Hire used negative years — both used BC years before their year zero. During the nineteenth century, both BC and negative years before year zero can be found. By the mid twentieth century, only negative years were being used before year zero.
ISO 8601:2000, but not ISO 8601:1988, explicitly uses a year zero (in the four digit form 0000) in its date reference system, which also includes the rules of the Gregorian calendar for all years, both before and after 1582, as well as negative years before year zero.
All eras used with Hindu and Buddhist 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. This is similar to the Western method of stating a person's age — people do not reach age one until one year has elapsed since birth (but their age during that first year is specified in months, not as age zero).
Many Maya scholars, but not all, assume (or used to assume) that a year zero exists in the modern calendar and thus specify that the epoch of the Long Count of the Maya calendar occurred in 3113 BC rather than 3114 BC.
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.
In the movie Back to the Future, Dr. Emmett Brown, the inventor of a time machine, enters the date of the "birth of Christ" on a keypad as December 25, 0000, and thus shows in three different ways that he is not a historian. Not only does he use the astronomical year number, but he assumes that Jesus was actually born in that year (he was born between 8 BC and AD 9, according to different sources), and also that Christianity's celebration of Christmas on December 25 is a true historical anniversary rather than a traditional date. (Of course, it's possible that he was just being funny and/or showing off.)
The famous and ficticious theologian Franz Bibfeldt's most famous work relates to the year 0: a 1927 dissertation submission to the University of Worms entitled "The Problem of the Year 0." [1]
