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.
Timeline of Christianity
The purpose of this timeline is to give a detailed account of Christianity from the beginning of the current era (AD) to the present. Question marks ('?') on dates indicate approximate dates.
The year one is the first year in the "Christian calendar" (there is no year zero), which is the calendar presently used (in unison with the Gregorian calendar) almost everywhere in the world. Traditionally, this was held to be the year Jesus was born; however, most modern scholars argue for an earlier or later date, the most agreed upon being between 6 BC and 4 BC.