Time of supply determines when GST becomes payable, based on invoice issue, payment receipt, or receipt of goods. The statute fixes the time of supply as the point when tax liability arises: for standard supplies the earlier of invoice issue (or last required issue date) and payment receipt, with payment deemed at book entry or bank credit, and an option to treat small excess receipts as invoiced. For reverse charge supplies the earliest of goods receipt, payment recorded/debited, or expiry of a specified period after invoice governs, reverting to recipient's books if indeterminate. Voucher supplies are timed on issue if identifiable or on redemption; otherwise return filing or tax payment dates apply. Value additions by interest, fees or penalties take effect when received by the supplier.