Article 21 addresses the tax calculation for supplies that span key transition periods. It stipulates that if an invoice is issued or payment is made for goods or services before the VAT law's effective date or the supplier's registration date, but the supply itself occurs after, then VAT is due on the actual date of supply. Paragraph 2 extends this rule to internal supplies between a taxable supplier in the Kingdom and a customer in another Member State. For continuous supplies, paragraph 3 clarifies that tax is not due on the portion of the supply rendered before the effective or registration date.