Time of supply determines when GST arises: invoice issuance, payment receipt, or accounting entry triggers tax liability for supplies. The Time of Supply fixes when GST liability arises: for goods the earlier of invoice issuance (or latest statutory invoice date) and payment receipt; for services the earlier of invoice (if timely) or provision and payment receipt, with the recipient's book entry serving where neither applies. Vouchers are supplied on issue if identifiable, otherwise on redemption. Residual rules use return due date or actual tax payment. Reverse charge and value additions have specific temporal triggers, and invoice timing and rate change interactions determine applicable tax rate.