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.
Year 1900 problem
When handling the year 2000 problem, measures were sometimes taken to avoid or rectify this: modifying the national identification number, for instance. For example, year is recorded only with two digits in Bulgarian Uniform civil number; however, a solution was ready as early as the inception of the system in 1975: 20 was added to the month number for individual born before 1900 and 40 for those born in or after 2000.