Year zero
Year zero does not exist in the anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year is followed by . However, there is a year zero in astronomical year numbering (where it coincides with the Julian year ) and in ISO 8601:2004 (where it coincides with the Gregorian year ) as well as in all Buddhist and Hindu calendars.
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.