Originally, Christian Era, BCE (Before Common Era), the calendar notation adopted by scholars in the Twentieth Century of the Common Era to designate the Year in place of AD (Anno Domini) and Before Christ (BC), respectively. Because of poor time calculation in earlier times, there may be anomalies in dating. Thus, the beginning of the Christian era, originally fixed probably by Dionysius Exiguus , was set a little too late. Therefore the actual birth of Christ must be dated earlier, probably in 4 BCE.
In chronology an era is a period reckoned from a fixed point in time, as before or after the birth of Christ—before Christ, BC; Anno Domini (year of the Lord), AD. Other points best known from Earth Western history are; 1) The creation of the world (Jewish, equivalent to 3761 BCE; Byzantine, 5508 BCE); 2) The founding of the city of Rome in 753 BCE; year marked 1 A.U.C. for ab urbe condita (from the founding of the city); 3) The Hegira , the flight of Muhammad from Mecca (AD 622; abbreviation AH); and 4) The founding of the Olympic games in ancient Greece (776 BCE; time in Olympiads).