I assume you are referring to the Anno Domini year numbering system, where history is divided into BC (Before Christ) and AD (Anno Domini). BC counts backwards from what was once believed to be the year of Jesus Christ's birth, while AD counts forward.

This system was divised by Dionysius Exiguus in the year 525 AD to replace an earlier system that made reference to reign of Emperor Diocletian. Dionysius did not wish to continue the memory of an emperor who had persecuted Christians.

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.

Nowadays, in order to remove the religious connotations from the numbering system, it is common to refer to the eras as BCE (Before Common Era) and CE (Common Era.) The year numbers convert exactly into the BC/AD system. It is simply a change of name.
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