"A.D." stands for anno domini, Latin for “in the year of the lord,” and refers specifically to the birth of Jesus Christ. In recent years, an alternative form of B.C./A.D. has gained traction. Many publications use "C.E.," or "common era," and "B.C.E.," or "before common era."
The B.C./A.D. system gained in popularity in the ninth century after Holy Roman Emperor Charlemagne adopted the system for dating acts of government throughout Europe.
By the 15th century, all of Western Europe had adopted the B.C./A.D. system. The system's inclusion was implicit in the 16th-century introduction of the Gregorian calendar, and it later would become an international standard in 1988 when the International Organization for Standardization released ISO 8601, which describes an internationally accepted way to represent dates and times.
Hope it helps...