Time of supply determines tax liability timing: generally invoice issue or payment receipt, with special rules for reverse charge and vouchers. Time of supply of goods arises when tax liability occurs and is typically the earlier of invoice issuance (or its statutory last date) or receipt of payment; for reverse-charge supplies it is the earliest of goods receipt, payment as entered or debited in the recipient's books, or the date after thirty days from supplier's invoice, with the recipient's books entry as fallback. Voucher supplies use voucher issue or redemption dates. If none of these rules apply, the time is the periodic return due date or the date of tax payment. Additions by interest, late fee or penalty are taxable when received.