If the Immaculate Conception was on December 8, how come Jesus was born on December 25? ?

Just wondering. If it takes 9 months for a baby to develop, and Mother Mary was conceived by the Holy Spirit on December 8, shouldn't Jesus' birthday be around September?

Thanks in advance!
Oh, this is not intended to insult Christianity in any way whatsoever.
Update: If ever you still don't get it, Immaculate Conception (Dec. 8): Mary Conceives Jesus. (Start of pregnancy, not Mary's birth, she was born on Sept. 8) Christmas (December 25): jesus' birth. Calculate 9 months from Dec. 8, will lead to around September 8. (Exactly 9 months.) So how come it's on... show more If ever you still don't get it,
Immaculate Conception (Dec. 8): Mary Conceives Jesus. (Start of pregnancy, not Mary's birth, she was born on Sept. 8)
Christmas (December 25): jesus' birth.
Calculate 9 months from Dec. 8, will lead to around September 8. (Exactly 9 months.)
So how come it's on December 25?
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