A due date is basically when the payment of an invoice or bill is due. The bill amount does not need to be in the billers account on the due date for the payment to be timely. You simply need to have initiated the payment on the due date for it to be paid on time. If you write a check, your have paid on time if you mail your check on the due date. If you pay via ACH or bank bill pay, you have paid on time if you initialize the transfer on the due date.