BC stands for Before Christ, and it’s a way of counting years before the estimated birth of Jesus Christ. AD starts with year 1 and continues to the present day. BC starts with the year before Christ’s birth, 1 BC, and counts backward to the year 10,000 BC. AD and BC provide a useful framework for historical and religious contexts; however, they’re not completely accurate. The accuracy of AD and BC is limited by the fact that they’re based on an approximation of the birth of Jesus Christ, which is itself not entirely certain.

Source: Grammarly