what is the differences between A.D and B.C

The AD/BC method of identifying historical dates can be traced back to Catholic historians working in the early Middle Ages. Identifying historical dates until that point was often a complicated proposition, since different historians worked under different calendars. A Roman historian would have used the Roman AUD notation, in which Year Zero was the largely symbolic founding of Rome. Converting historical dates to the standard Gregorian calendar would not have been easy. Using the birth of Jesus Christ as a central point made more sense to the religious historians.

The term BC is short for "Before Christ." Historical dates before the birth of Christ become smaller as they approach the theoretical but non-existent Year Zero. Historical dates after the birth of Christ are classified as AD, short for the Latin phrase Anno Domini, or "in the year of our Lord." Contrary to popular belief, AD does not stand for "After Death." The BC/AD system for identifying historical dates has been in continuous use ever since the earliest part of the Middle Ages, at least.