September 11th comes from the idea that Revelation 12:1-4 was a set of astrological clues that may have been how the Medjay (Magi) found the baby Yeshuah in the manger in Bethlehem. Dr. Michael Heiser talked about it on the Naked Bible Podcast. I think it was September 11, of the year 3 B.C. was the exact date based on computer astronomy models getting rewound. I think September 15, of the year 7 B.C. was the other date that people felt qualified.

The year 0 was a pretty solid guess by a 6th century monk, Dionysius Exiguus, but there's almost no way he was right working with what would have been available 500 years after Christ. It was laudable but not perfect.