Time of supply determines when GST is payable: earliest of receipt of payment, invoice issuance, or supply completion. The timing for GST payment is governed by the time of supply rules: for goods under Section 12 and for services under Section 13. GST is payable at the earliest of three events - receipt of payment, issuance of the invoice, or completion of the supply - and which event applies depends on the factual situation to determine the tax point.