Inclined to agree with Peter C
If in any doubt, always charge interest from the later period specified.
In this instance, the OC will act like any other corporate body which does not specifiy credit provision/default terms above and beyond the minimum ‘interest payable’ clauses.
The general tests for charging ad-hoc fees and interest payments is whether the business (in this case the OC), by going about collecting the unpaid monies, actually incurred a cost in doing so. As you would not normally have started collecting monies during the grace period, then the answer is that you would not be in a position to charge for it.