Anno Domini - No Year Zero

No Year Zero

Further information: 0 (year), Astronomical year numbering, and Millennium

In the AD year numbering system, whether applied to the Julian or Gregorian calendars, AD 1 is preceded by 1 BC. There is no year "0" between them. Because of this, most experts agree that a new century begins in a year with the last digits being "01" (1801, 1901, 2001); new millennia likewise began in 1001 and 2001. A common misconception is that centuries and millennia begin when the trailing digits are zeroes (1800, 1900, 2000, etc.); moreover, this convention was widely used to celebrate the new millennium in the year 2000. For computational reasons astronomical year numbering and the ISO 8601 standard designate years so that AD 1 = year 1, 1 BC = year 0, 2 BC = year −1, etc. In common use, ancient dates are expressed in the Julian calendar, but ISO 8601 uses the Gregorian calendar and astronomers may use a variety of time scales depending on the application. Thus dates using the year 0 or negative years may require further investigation before being converted to BC or AD.