Anno Domini (Latin: "In the Year of the Lord"), or more completely Anno Domini Nostri Iesu Christi ("In the Year of Our Lord Jesus Christ"), commonly abbreviated AD or A.D., is the designation used to number years in the dominant Christian Era in the world today.
...The Anno Domini system was developed by a monk named Dionysius Exiguus (a Scythian) in Rome in 525, as an outcome of his work on calculating the date of Easter.
...Almost all Biblical scholars believe that Dionysius was incorrect in his calculation, and that the date claimed for Jesus' birth was between 8 BC and 4 BC. The latest bound for the birth of Christ is the death of Herod the Great which occurred in 4 BC.
In case you're wondering, I have no idea what "bound" means in that context. It must be a typo at the Wikipedia site.