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.
Library of Congress Control Number
In its most elementary form the number includes a year and a serial number. The year has two digits for 1898 to 2000, and four digits beginning in 2001. The three ambiguous years (1898, 1899, and 1900) are distinguished by the size of the serial number. There are also some peculiarities in numbers beginning with a "7" because of an unsuccessful experiment applied between 1969 and 1972.