In the EPC, the term "due date" has a special meaning, namely the first day on which payment of a fee may be validly effected, not the last day of a period for such payment (see A‑X, 6 "Payment in due time"). The due date for fees is generally laid down by provisions of the EPC or of the PCT. If no due date is specified, the fee is due on the date of receipt of the request for the service incurring the fee concerned.
A fee may not be validly paid before the due date. The only exceptions to that principle are:
Payments made before the due date which are not valid may be refunded by the EPO. If payment is made shortly before the due date, it is possible that the EPO will not return the payment. In this case, however, payment only takes effect on the due date. See A‑X, 5.2.4 regarding renewal fees for a European patent application.