Chronology, the science of measuring time. Chronology divides time into regular divisions or periods, and assigns events their proper place and sequence by giving them dates. Some striking event or change is chosen as the starting point in measuring time. This point is an epoch, and it begins a period called an era.
The system now almost universally used with the birth of Christ as the epoch that begins the erawas established by Dionysius, a sixth-century Italian monk, and popularized by Saint Bede, an eighth-century English scholar. Dionysius introduced the concept of A.D. (Latin, Anno Domini, in the year of our Lord) for dates in the Christian Era and Bede introduced B.C. (before Christ) for dating events occurring before that era. (Some modern writers use C.E. and B.C.E., for common era and before the common era, in place of A.D. and B.C.) Most scholars believe that Christ was born several years earlier than Dionysius calculated. Thus his birthdate is given as between 8 B.C. and 2 B.C.
The epoch of Jewish chronology is the date on which, according to Jewish tradition, the universe was created. It corresponds to 3761 B.C. The ancient Greeks reckoned time from the beginning of the Olympic games, traditionally placed in 776 B.C. Their era was divided into Olympiads, the four-year intervals between games. The Roman epoch was the traditional founding of Rome in 753 B.C. Muslim chronology begins with the Hegira (the flight of Mohammed from Mecca), which occurred in 622 A.D.
Such sciences as geology and archeology have established chronologies covering billions of years.