In the dating system used for calendars, 622 CE refers to the year 622 of the Common Era. It is equivalent to saying 622 A.D. In the past, the convention was to use B.C, and A.D. because calendars were based on the birth of Christ. To represent a time before his birth, B.C. was used, and A.D meant a time after his birth. Historians have adopted an alternative system to designate time on a calendar, using instead the CE and B.C.E.