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
The "anno Domini" nomenclature was not widely used in Western Europe until the 9th century, and the to 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.