It was the beginning of the Christian era/common era. The preceding year is 1 BC; there is no year 0 in this numbering scheme. The Anno Domini dating system was devised in AD 525 by Dionysius Exiguus.AD 1. Millennium:1st millennium Decades:10s BC 0s BC 0s 10s 20s Years:3 BC 2 BC 1 BC AD 1 AD 2 AD 3 AD 4