Throughout much of the world, calendars as well as encyclopedias use the Gregorian calendar, introduced by Pope Gregory XIII in 1582. By this system, historical years are counted since the (assumed) year of the birth of Jesus Christ, a tradition that began in the sixth century. By tradition, years since his birth are followed by A.D. for "anno Domini," or "year of our Lord."
By the same tradition, years before the birth of Christ are followed by "B.C." or "before Christ" in English. In this system, the year 1 B.C. is followed immediately by A.D. 1; there is no year 0. This system has a built-in problem: There is no single year specified as the year of Christ's birth; all years are either "before" or "after."
Since the early 19th century, a more recent pair of abbreviations has gradually come into use. The system grew popular among Jewish historians. Many encyclopedias now accept this system to designate historical years. The pair consists of "BCE," which stands for "Before Common Era" or "Before Christian Era," and "CE," for "Common Era" or "Christian Era." The system of numbering years remains the same, but "CE" replaces "A.D.," and "BCE" substitutes for "B.C."
As the world has become more interconnected, the use of CE and BCE has spread. Many historians and encyclopedia editorial boards, prefer these abbreviations as a way of relying on the globally accepted Gregorian system of numbering years without direct reference to Christianity. The use of B.C. and A.D., from this viewpoint, is an anachronism when applied to histories and cultures of other religions.
Many scholars also see CE and BCE as simply more accurate labels. The specific year of the birth of Christ is still a subject of debate. And when applied to centuries, "A.D." doesn't make any sense, because the literal translation from the Latin original is "year" of our Lord.