And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
I can't explain the math so I am linking to a short article that does a much better job than I ever could of explaining mathamatically how we actually have really good indicators of when Jesus was actually concieved & born & having those indicators how much more amazing & wonderful & holy & all of a piece with Scripture Jesus birth actually is & why Christmas has so diminished the wonder & the glory that it actually is.
Symbolically this makes much better sense to me than anything the church has come up with. I can hear the Spirit of God in this thinking because this is how I experience God also: symbolically consistent from beginning to end. If you study Passover you see exactly the same symbolic consistency. I can't think of a festival where this is not so.
It is beautiful in deeply meaningful ways & it is like a many faceted diamond. No matter which way you look at it the light magnifies & glories God. And that, my friends, is the Spirit's job; to turn us & keep turning us towards Christ!