I assume you are referring to the __Anno Domini year numbering system__, where history is divided into __BC__ (Before Christ) and __AD__ (Anno Domini, or “in the year of our Lord”, sometimes mistakenly referred to as AC or “After Christ”). 

These terms are used to label or number years in the Julian and Gregorian calendars, based on the traditionally reckoned year of the conception or birth of Jesus Christ. 

This dating system was devised in 525 AD by Dionysius Exiguus, a Christian monk, to replace an earlier system that made reference to the reign of Emperor Diocletian, who had persecuted Christians. Dionysius did not wish to continue the memory of such a tyrant.

Having a numbering system with two distinct eras has many advantages. It represents a simplification of systems that number years in reference to a reigning monarch or office holder, as it doesn’t require one to be familiar with all such individuals and the time they reigned. BC and AD, collectively, cover all past and future years by counting in both directions from a single reference point.
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