Year zero
Historians have never included a year zero. This means that between, for example, and , there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage "anno Domini" 1 is preceded by the year 1 BC, without an intervening year zero. Neither the choice of calendar system (whether Julian or Gregorian) nor the era ("Anno Domini" or Common Era) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.
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.