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.
Red Eyes
The year 2001 became the new first year of the new era, known as 1 GU (first year of the Great Unification's calendar), however in 50 GU (2050 AD) an unknown power is able to hack in the SDI system, forcing the system in SafeMode, making the system unable to process order from the DUPE command centre (except its original mission to intercept any launched ICBMs and missiles). As result of losing its deterrence system, the other countries of the planet launch a full-scale attack against the DUPE forces, beginning the "20 years war" (World War III) that ended de facto in 70 GU (2070 AD) with no winner and the death of half the population of the planet, while all main actors of the war eventually dissolved and ceased to exist following the mutual destruction.