I would have thought the interest would be charged from the due date unless payment is received before the end of the grace period. Nonetheless, since there is uncertainty I would suggest the EC should start charging interest from the later date. That way an incentive for payment still exists but you won’t get tied up in arguments about which date to start from; the late payer is unequivocally in the wrong and the OC clearly right and possibly even generous. One month either way is not much difference when the real problem for some OCs are the people who don’t pay for many months.