Year zero
Historians have never included a year zero. This means that between, for example, and , 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. 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.
Year zero
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.