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.