era, period of historic time. In geology, it is the name applied to large divisions of geological process, e.g., Paleozoic era (see geology). In chronology an era is a period reckoned from a fixed point in time, as before or after the birth of Christ—before Christ, B.C.; Anno Domini [year of the Lord], A.D. The points best known for Western history are the creation of the world (Jewish, equivalent to 3761 B.C.; Byzantine, 5508 B.C.); the founding of the city of Rome [753 B.C.; year marked A.U.C. for ab urbe condita (from the founding of the city)]; the Hegira, the flight of Muhammad from Mecca (A.D. 622; abbreviation A.H.); and the founding of the Olympic games in ancient Greece (776 B.C.; time in Olympiads). Some people use C.E. (originally, Christian era, now common era) and B.C.E (before common era) in place of A.D. and B.C., respectively. Since in different calendars years are of different lengths and do not begin on the same day (see calendar), several factors have to be used in changing the year of one era to that of another, and even with conversion charts there are still difficulties. 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 Jesus must be dated a little earlier, probably in 4 B.C. The term epoch is often confused with era in writing.