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.
Panic in Year Zero!
After a harrowing journey, the Baldwins reach their destination and find shelter in a cave, while they wait for order to be restored. On their portable radio they listen to war news and learn what is left of the United Nations has declared this to be "Year Zero". Harry and Rick find that Johnson and his wife are their neighbors, but not for long; they are killed by the thugs.