Invoices given and payments made on or before appointed day

(1) Notwithstanding section 18, where on or before the appointed day, an invoice is given or payment is made for a supply of goods or services, tax shall be payable on such supply only to the extent that any part of the supply as is made after the (2) For the purposes of this section, the time of a supply of goods or services referred to in sub-section (1) shall be the time when the goods are made available or the services are rendered, and where the supply is a progressive or periodic supply, section 18(3), shall apply for the purposes of sub-section (1).