Year zero
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"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1. However, there is a year zero in astronomical year numbering (where it coincides with the Julian year 1 BC) and in ISO 8601:2004 (where it coincides with the Gregorian year 1 BC) as well as in all Buddhist and Hindu calendars.
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Counting intervals without a zero
The absence of a year 0 leads to some confusion concerning the boundaries of longer decimal intervals, such as decades and centuries. For example, the third millennium of the Gregorian calendar began on 1 January 2001, rather than the widely celebrated 1 January 2000. Likewise, the 20th century began on 1 January 1901.
This rule results from the fact that the Gregorian calendar begins with a year 1 instead of 0. Cardinal and ordinal numbering of years is therefore identical: The year 10 is the tenth year of the calendar and the end of the first decade. The year 11 is the first year of the second decade, and so on. In spite of this rule, years ending in 0, rather than 1, are commonly perceived as marking the beginning of a new decade, century, or millennium.
Historical, astronomical and ISO year numbering system
Historians
The Anno Domini era was introduced in 525 by Dionysius Exiguus (c.470–c.544), who used it to identify the years on his Easter table. He introduced the new era to avoid using the Diocletian era, based on the accession of Emperor Diocletian, as he did not wish to continue the memory of a persecutor of Christians. In the preface to his Easter table, Dionysius stated that the "present year" was "the consulship of Probus Junior [Flavius Probus]" which was also 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown.
Dionysius did not use AD years to date any historical event. This began with the English cleric Bede (c. 672–735), who used AD years in his Historia ecclesiastica gentis Anglorum (731), popularizing the era. Bede also used a term similar to the English before Christ once, but that practice did not catch on until very much later. Bede did not sequentially number days of the month, weeks of the year, or months of the year, however, he did number many of the days of the week using a counting origin of one in Ecclesiastical Latin. Previous Christian histories used anno mundi ("in the year of the world") beginning on the first day of creation, or anno Adami ("in the year of Adam") beginning at the creation of Adam five days later (the sixth day of creation according to the Genesis creation narrative), used by Africanus, or anno Abrahami ("in the year of Abraham") beginning 3,412 years after Creation according to the Septuagint, used by Eusebius of Caesarea, all of which assigned "one" to the year beginning at Creation, or the creation of Adam, or the birth of Abraham, respectively. Bede continued this earlier tradition relative to the AD era.
In chapter II of book I of Ecclesiastical history, 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. The modern English term "before Christ" (BC) is only a rough equivalent, not a direct translation, of Bede's Latin phrase ante incarnationis dominicae tempus ("before the time of the lord's incarnation"), which was itself never abbreviated. Bede's singular use of 'BC' continued to be used sporadically throughout the Middle Ages.
It is often incorrectly stated that Bede did not use a year zero because he did not know about the number zero. Although the Arabic numeral for zero (0) did not enter Europe until the eleventh century, and Roman numerals had no symbol for zero, Bede and Dionysius Exiguus did use a Latin word, nulla meaning "nothing", alongside Roman numerals or Latin number words wherever a modern zero would have been used.
The anno Domini nomenclature was not widely used in Western Europe until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in Fasciculus Temporum by Werner Rolevinck in 1474, alongside years of the world (anno mundi). The terms anno Domini, Dionysian era, Christian era, vulgar era, and common era were used interchangeably between the Renaissance and the 19th century, at least in Latin. But vulgar era was suppressed in English at the beginning of the 20th century after vulgar acquired the meaning of "offensively coarse", replacing its original meaning of "common" or "ordinary". Consequently, historians regard all these eras as equal.
Historians have never included a year zero. This means that between, for example, January 1, 500 BC and January 1, AD 500, there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage anno Domini 1 is preceded by the year 1 BC, without an intervening year zero. Thus the year 2006 actually signifies "the 2006th year". Neither the choice of calendar system (whether Julian or Gregorian) nor the era (Anno Domini or Common Era) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood. No historian includes a year 0 when numbering years in the current standard era.
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