What's so important about the time (e.g., the date) of birth?

You've probably already considered a good number of theories as to why we choose this date, but let me share my own, if even to reinforce yours.

Some couples are very sexually involved, and doctors cannot track the day of conception in many cases, so could not be used without only estimating.

In a lot of cultures, people believe that the soul enters the body upon birth, therefore the *real* birth is actually occurring at the time the baby leaves the womb.

Another possibility may be that when people chose to make the time of birth, however long ago that may be, they did not understand that unborn babies are already so mentally developed. Maybe they thought the fetuses were just blank slates until they were born.

Or maybe, what I'm betting on, is that someone somewhere *just decided* to do it that way, so that is how we do it now.

A conception isn't necessarily a birth, for many reasons. The technical time of birth for astrologers is when the first breath is taken. This is when the soul comes into the body and makes a commitment to this life.