Even though we don't know the exact historical date of the birth of the Lord Jesus, we count the years as if we did.
So in a few days we are going to begin the Year of Our Lord 2007.
The correct use of "A.D." is to place it BEFORE the number of the year, as in: A.D. 2007.
The CORRECT placement thus gives: "in the year of the Lord 2007".
If you do it incorrectly and backwards, as in "2007 A.D." the effect is literally "2007 in the year of the Lord"-- which is nonsense.
By contrast, the use of "B.C." ("Before Christ") requires that you place the "B.C." AFTER the number of the year, as in: 1000 B.C.-- meaning "1000 (years) before Christ."
Since we are theoretically counting the years since the birth of Christ, then why doesn't the year begin with December 25, the day we celebrate his birthday?
After all, the calendar as we've used it for centuries was promulgated by one of the popes.
Here's how I see it.
We designate December 25 as the birthay of Christ.
However, we liturgically celebrate his birthday as an octave-- an observance of eight days.
So, liturgically speaking, the final "moment" of the birthday of Christ is January 1-- and that's when we begin counting the new "Year of the Lord."