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.
Timeline of historical geopolitical changes
This article uses the Common Calendar System, which was established as the new Christian Calendar by Pope Gregory XIII in 1582. The Common Calendar uses the designation Common Era (CE or AD for Anno Domini) for years starting 1 January 1 CE, and Before Common Era (BCE or BC for Before Christ) for years before that date. The Common Calendar also follows the ordinal numbers rather than the cardinal numbers, so there was no "year zero" in this format; that is, the date 1 January 1 CE immediately followed the date 31 December 1 BCE. For the same reason, while the 2000s began on Saturday, 1 January 2000 CE, the official Third Millennium began on Monday, 1 January 2001 CE.