Editing Anno Domini (section)

== No year zero: start and end of a century ==
{{Further|Year zero|Astronomical year numbering|Millennium|Century|Decade}}

In the AD year numbering system, whether applied to the [[Julian calendar|Julian]] or [[Gregorian calendar]]s, AD 1 is immediately preceded by 1 BC, with nothing in between them (there was no [[year zero]]). There are debates as to whether a new decade, century, or millennium begins on a year ending in zero or one.<ref name="Teresi1997" />

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.{{efn|To convert from a year BC to [[astronomical year numbering]], reduce the absolute value of the year by 1, and prefix it with a negative sign (unless the result is zero). For years AD, omit the AD and prefix the number with a plus sign (plus sign is optional if it is clear from the context that the year is after the year 0).<ref>Doggett, 1992, p. 579</ref>}} In common usage, 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.