Time of supply for services determines when tax arises, covering invoice, payment, reverse charge and voucher timing. The time of supply of services determines tax liability and is the earliest of invoice issue (when timely), provision of service (if invoice delayed), or receipt of payment; failing these, the recipient's book entry controls. Reverse charge supplies, voucher supplies, and value additions (interest, late fee, penalty) have specific timing rules, with fallbacks to recipient accounting entries, return filing dates or tax payment dates where standard tests cannot determine timing.