Year zero
Year zero does not exist in the anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year is followed by . However, there is a year zero in astronomical year numbering (where it coincides with the Julian year ) and in ISO 8601:2004 (where it coincides with the Gregorian year ) as well as in all Buddhist and Hindu calendars.
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.