Time of supply of services determines tax liability based on invoice, payment, provision, or recipient books. Liability to tax services arises at the time of supply, defined as the earliest of invoice issue (if timely), receipt of payment, provision of service (when invoice late), or the recipient's book entry; a supplier may elect to treat small excess receipts as supplied on invoice date. Under reverse charge, time of supply is the earlier of recipient payment or the date following a statutory period after invoice, with recipient's books as a fallback; for cross-border associated enterprises, recipient books or payment govern. Vouchers are supplied on issue if identifiable or on redemption. If none apply, the return filing date or tax payment date determines time of supply, and additions like interest are timed on receipt.