Year zero
ISO 8601:2004 (and previously ISO 8601:2000, but not ISO 8601:1988) explicitly uses astronomical year numbering in its date reference systems. Because it also specifies the use of the proleptic Gregorian calendar for all years before 1582, some readers incorrectly assume that a year zero is also included in that proleptic calendar, but it is not used with the BC/AD era. The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: −0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an absolute value one less than the equivalent BC year, hence -0001 = 2 BC. Because only ISO 646 (7-bit ASCII) characters are allowed by ISO 8601, the minus sign is represented by a hyphen-minus.
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.