Time of supply of goods determines tax liability: invoice or payment triggers tax, with special rules for reverse charge and vouchers. The liability to pay tax on goods arises at the time of supply, defined as the earlier of invoice issuance (or last allowed invoice date) and receipt of payment, with supply deemed to the extent covered by invoice or payment and receipt recorded by books or bank credit. For reverse charge supplies the time of supply is the earliest of receipt of goods, payment as recorded by the recipient, or lapse of a prescribed period after invoicing; fallback rules apply where these events cannot be determined. Voucher supplies are timed to issue or redemption, and additions to value for interest, late fees or penalties are timed to receipt.