Dionysis the Little
Dionysis the Little was a Roman monk in the sixth century AD. He is famous because he changed the calendar to recognize the birth of Christ. He calculated that Jesus was born the year that King Herod died, and modern historians agree. However, the monk thought that Herod died 753 years after the founding of Rome, instead of the correct 749 years after the founding of Rome. (King Herod is the ruler who ordered all infants in Bethlehem younger than 2 to be killed in his unsuccessful attempt to murder Jesus, whose family fled to Egypt to escape this.) Thus the monk estimated that Jesus was born four years later than He probably was. What was intended to be A.D. 1 was actually 4 B.C.
Note that there is no year "0", as the concept of "0" was unknown to the Greeks and not even discovered until the seventh century by mathematicians in India. The year that follows 1 B.C. is A.D. 1; i.e., the year that follows the first year before Christ is the first year of our Lord.