best time for hubbie to take paternity leave?

hi im only 5 months so this might be a bit premature..but planning ahead and thinking about hubbies paternity leave, when is the best time to take this?as far as i can see he can only take it starting on my due date or after..but what happens if baby is born before my date? or what if i am left 2 weeks overdue, as can happen in my hospital and then he's away back to work?can he just give a rough estimate then just be off from whenever baby is born? is anyone in HR or anything or knows about this?