We have a problem, Henry. If, as u believe, the human body is endowed with a soul and this soul 'enters' the body of the childrens before they are born, and the soul of the human being is the identity of the person, then a person's birfday is not the day they were born.
Therefore, we need to determine two things first. At what point did Henry's body become endowed with him (his soul), and what day of what month that point happened.
The hard part is figuring out when during the growth of cells does Henry hit the scene. We have a nine month window. Your birfday will be somewhere in there.