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The AD/BC notation does not use a year zero; however, astronomical year numbering does use a minus sign, so '2 BC' is equal to 'year -1', '1 BC' is equal to astronomical 'year 0', and so forth. In the Gregorian system, the Nth century AD ..starts in the year (N-1)*100+1 and .. ends in year N*100, so for example, the 16th century begins on January 1, 1501 and ends December 31, 1600. However, year order is reversed in the BC era, < so the Nth century BC begins in the year N*100 BC and ends in (N-1)*100+1 BC, so for example the third century BC begins in 300 BC and ends 201 BC. |