is produced by multiplying together the lunar cycle, the solar cycle, and the Roman indiction. The first year of the Christian era corresponded to the year 4713 of the Julian, and therefore to reduce our B.C. dates to the Julian, we must subtract them from 4713, but our A.D. dates we must add to that number. So named from Julius Scaliger, the deviser of it.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894