Perhaps this is deliberate we have been looking to pin some predictable, astronomical phenomena to Harry’s birth to get an exact date but what if there isn’t? What if it was set up as dawn/sunset on a random day which we just adopted as 31 October/Halloween by humanity over history, and the 666 interval just mathematics? We never then get an exact date of birth and don’t need one, which is just as well as Jim won’t give one, and he is happy with us chasing are tails over “clues” which are little more than red-herrings.
The top tier beings responsible for this pre-date linear time which has been hammered into us, to them one date was as good as any other.
Balance is a big thing in the Dresdenverse. What if Harry was born at dawn, and Listen born at Sunset? That would be very dramatic, we have Bob and Evil Bob, Mouse and Ash, why not Harry and Listen as a pairing (not siblings)