A.D. is the abbreviation for anno Domini, which is Latin for in the year of our Lord. In 532, the monk Dionysius Exiguus introduced a system of dating events, beginning with the year he believed Jesus Christ was born. In this system, the year of Christ’s birth was A.D. 1, and the year before that was 1 B.C. (before Christ). Modern scholars believe Christ was actually born no later than 1 B.C. But people still determine dates using the original system.
Writers generally use A.D. and B.C. only to avoid confusion. When a writer mentions a year without using either abbreviation, readers should assume that the year was, or is, an “A.D.” year.
Because there is no “year zero” in the dating system, the calculation of an interval between a date in a “B.C.” year and the same date in an “A.D.” year requires two steps: First, add the numbers representing the years. Then subtract 1. Thus, the interval between the end of 1 B.C. and the end of A.D. 1 was 1 year. The interval between the end of 2000 B.C. and the end of A.D. 2000 was 3,999 years.
An alternative system uses the same numbering method as that of Dionysius Exiguus, but does not refer specifically to Christ. In the alternative system, C.E., which stands for common era, replaces A.D.; and B.C.E. (before the common era) replaces B.C.
