The Christian Era was introduced about the year 527 by Dionysius Exiguus, a monk resident at Rome, who fixed its starting point in the year 753 from the foundation of Rome, in which, according to his calculation, the birth of Christ occurred. By making this the year 1 of the Christian Era, he counted the years which followed in regular course from it, calling them years "of the Lord", and we now designate such a date A.D. (i.e. Anno Domini). The year preceding A.D. 1 is called Ante Christum (A.C.) or Before Christ (B.C.). It is supposed by many that the calculation of Dionysius was incorrect, and that the birth of Christ really occurred a few years earlier than he placed it. This, however, is immaterial for the purposes of chronology. It is important to understand that from a historical point of view is not possible to know exactly when Jesus was born – neither the year, the month, nor the day, so this discrepancy is only normal.