Time of supply determines when service tax liability arises; invoice, payment, voucher redemption or recipient records govern timing. The time of supply determines when service tax liability arises and is the earliest relevant event: invoice issuance within the prescribed period, receipt of payment, provision of service if invoice not timely, or recipient book entry. Reverse charge supplies use the earlier of recipient payment or a fixed post-invoice period, with fallback to recipient records; supplies by non resident associated enterprises rely on recipient entry or payment. Vouchers are timed by issue or redemption. If none of these tests apply, the time is the periodic return date or tax payment date. Additions like interest or late fees are taxable when received by the supplier.