A monk called Dionysius Exiguus (early sixth century A.D.) invented the dating system most widely used in the Western world. For Dionysius, the birth of Christ represented Year One. He believed that this occurred 753 years after the foundation of Rome.A monk called Dionysius Exiguus

Who decided Year 0?

In 1702 the French astronomer Philippe de la Hire labeled a year as Christum 0 and placed it at the end of the years labeled ante Christum, (BC), and immediately before the years labeled post Christum, (AD), on the mean motion pages in his Tabulæ Astronomicæ, thus adding the number designation 0 to Keplers Christi.