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.
Year zero
The "anno Domini" nomenclature was not widely used in Western Europe until the 9th century, and the to historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in "Fasciculus Temporum" by Werner Rolevinck in 1474, alongside years of the world ("anno mundi"). The terms "anno Domini", "Dionysian era", "Christian era", "vulgar era", and "common era" were used interchangeably between the Renaissance and the 19th century, at least in Latin. But "vulgar era" was suppressed in English at the beginning of the 20th century after "vulgar" acquired the meaning of "offensively coarse", replacing its original meaning of "common" or "ordinary". Consequently, historians regard all these eras as equal.