Time of supply of goods determines tax liability, triggered by invoice issuance or receipt of payment as primary events. The time of supply of goods fixes tax liability and is generally the earlier of invoice issue (or required issue) and receipt of payment, with supply limited to the extent covered by the invoice or payment and payment dated by book entry or bank credit. For reverse charge supplies, time of supply is earliest of receipt of goods, payment recorded or debited, or the date after thirty days from supplier's invoice; fallback to book entry if indeterminate. If none apply, time is the return due date or date tax is paid; additions like interest are timed when received by supplier.