Template:Centuriesdatenote

 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 NNth century AD starts in the base year 'NN-100+1' and ends in the 'NN'th year, so for example, the 16th century begins on January 1st, 1501 and ends December 31st, 1600.

However, year order is reversed in BC since there is no year zero and no century zero creating a ninety-nine year shift'', so the calculation is 'Nth century BC' begins in year 'N BC' and ends in 'N-99 BC' so that the third century BC, begins in 300 BC and ends 201 BC.