exactly as Tommy boy said...
sometimes due date is the same as it would be basically when their office closed.
I.e if their office closes at 4:30 then all work submitted has to be in by that time.
if you can email work until midnight, and their office closes at 4:30, then you get an extra 7 1/2 hours to work on the assignment, most teachers (rightly) consider that this is unfair to people who work on paper.