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.
Indian numerals
The addition of zero as a tenth positional digit is documented from the 7th century by Brahmagupta, though the earlier Bakhshali Manuscript, written sometime before the 5th century, also included zero. But it is in Khmer numerals of modern Cambodia where the first extant material evidence of zero as a numerical figure, dating its use back to the seventh century, is found.