Astronomical year numbering
Cassini gave the following reasons for using a year 0:
Fred Espanak of NASA lists 50 phases of the moon within year 0, showing that it is a full year, not an instant in time. Jean Meeus gives the following explanation:
Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantine historian Venance Grumel used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table. He did so possibly to save space and put no year 0 between them.
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.