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.
New Year
March 1 was the first day of the numbered year in the Republic of Venice until its destruction in 1797, and in Russia from 988 until 1492 (Anno Mundi 7000 in the Byzantine calendar). September 1 was used in Russia from 1492 (A.M. 7000) until the adoption of the Anno Domini notation in 1700 via a December 1699 decree of Tsar Peter I.