Time of supply of services determines when tax liability arises under invoice, payment, redemption, reverse charge and voucher rules. The time of supply of services is the earliest qualifying event among invoice issuance, receipt of payment, provision of service, or recipient recording; suppliers may treat minor excess receipts as taxable at invoice date for that excess. Reverse charge supplies use the earlier of recipient payment entry or a statutory fallback period after invoice, with accounting entry as further fallback. Voucher supplies are timed at issuance if identifiable or at redemption otherwise; unresolved cases default to return filing or tax payment dates.