Time of supply of services determines when tax liability arises, using invoice, payment, provision, or accounting triggers. Determination of the time of supply fixes when service tax liability arises: the earliest of invoice issuance (if timely), provision of service (if invoice delayed), or receipt reflected in recipient's books; payment is the earlier of accounting entry or bank credit. For reverse charge, time of supply is the earlier of recipient's payment or the date following a specified period after invoice; failing that, the recipient's accounting entry governs. Vouchers, cross-border associated enterprise supplies, return-filing periods, and receipt of interest or penalties have specified timing rules.