Time of supply of goods determines tax liability timing: earlier of invoice issuance or receipt of payment governs tax event. Time of supply of goods fixes tax liability at the earlier of invoice issuance (or its last permissible date) and receipt of payment, with payment treated as the earlier of accounting entry or bank credit. Reverse charge supplies use the earliest of goods receipt, recipient's payment entry or debit, or the day after thirty days from supplier's invoice, with recipient's books as fallback. Vouchers are taxable on issue if identifiable or on redemption otherwise; where none apply, time of supply is the periodical return date or tax payment date. Additions to value are taxable when received.