Time of supply rules determine when GST on services arises, prioritising invoice issuance, payment, or provision. The liability to pay GST on services arises at the time of supply, defined as the earliest of invoice issuance (within prescribed period) or receipt of payment, or provision of service where invoice timing differs; if none apply, the recipient's book entry controls. For reverse charge supplies the earlier of payment (by books or bank) or the date after sixty days from invoice governs, with book entry and special cross border fallbacks. Voucher supplies are timed at issue if identifiable or at redemption. Value additions (interest, late fee, penalty) crystallise when received by the supplier.