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.
Missing years (Jewish calendar)
Today, Hebrew dating places the creation of the world near the end of "Year One" AM and afterwards the first year of Adam's life as "Year Two" AM. However, Seder Olam Rabba shows that the Hebrew dating originally counted the first year of Adam's life as "Year Zero" AM. This may mean that the Hebrew dating has shifted in the course of history such that traditional dating of ancient events appears two years earlier than the modern Hebrew dating would be.