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===Dating systems===
{{Main|Year zero}}
In the [[Before Christ|BC]] [[calendar era]], the year 1{{nbsp}}BC is the first year before AD{{nbsp}}1; there is not a [[year zero]]. By contrast, in [[astronomical year numbering]], the year 1{{nbsp}}BC is numbered 0, the year 2{{nbsp}}BC is numbered −1, and so forth.<ref>{{Cite book |last=Steel |first=Duncan |url=https://archive.org/details/markingtimeepicq00stee_0/page/113 |title=Marking Time: The epic quest to invent the perfect calendar |publisher=John Wiley & Sons |year=2000 |isbn=978-0-471-29827-4 |page=[https://archive.org/details/markingtimeepicq00stee_0/page/113 113] |quote=In the B.C./A.D. scheme there is no year zero. After 31 December 1{{nbsp}}BC came 1 January AD 1. ... If you object to that no-year-zero scheme, then don't use it: use the astronomer's counting scheme, with negative year numbers. | url-access=registration|oclc =1135427740 }}</ref>