It’s obvious that Jesus was born on December 25, A.D. 1 , right? Wrong. We do know that Herod the Great (who killed all the babies in Bethlehem younger than 2 years of age) died in the Spring of 4 B.C., and the king was quite alive during the visit of the Wise Men (Magi) in the Nativity story told in the Gospel of Matthew. So Jesus must have been born before this time, anywhere from 7-4 B.C. (Before Christ, or before himself!)
Why is there a gap of this much time in our modern calendar? We owe this to a Roman monk-mathematician-astronomer named Dionysis Exeguus (Dennis the Little). During the 6th century he unwittingly committed what has become history’s greatest numerical error as it relates to the calendar. As he endeavored to reform the Western calendar to center around Jesus’ birth, he erroneously placed the date of the Nativity in the year 753 from the founding of Rome (753 a.u.c. or Ab Urbe Condita), even though Herod died only 749 years after the founding of the city of Rome. The cumulative effect of Dionysis’ calendar error, which is the same calendar we use today, was to give the correct traditional date for the founding of Rome, but one that is at least 4 to 7 years off for the birth of Christ.
Bill Petro, your friendly neighborhood historian
Inspired by Paul L. Maier’s In the Fullness of Time