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.
Flanaess
Gygax calculated all events that had taken place before 1 CY as negative numbers, and since there was no 0 CY—the year previous to 1 CY was -1 CY—this forced an extra calculation when attempting to convert Common Year dates prior to 1 CY to the other calendar systems. However, by the start of Gygax's campaign (set as 576 CY), the Common Year Reckoning was used almost universally by most cultures, making conversions to the other calendar systems by dungeon masters largely unnecessary.