The baby is due October 3!

We resolve to 0 if he's born exactly on the due date. Other examples:

• October 7 = 4 (4 days late)

• October 2 = -1 (1 day early)

We'll go by the date on the official birth certificate.

If the baby is born outside the provided range, the question resolves to the nearest number.

Clarification: The question says

We resolve to 0 if he's born exactly on the due date.

I've noticed that when resolving, I'll actually be forced to choose a range like 0-1 or 1-2. Therefore I will define 0-1 as October 3, I think that's most intuitive. You could think of it as the due date being October 3 midnight, and the range meaning 0.0 days (inclusive) to 1.0 days (exclusive).