Time of supply of services determines GST liability based on invoice, payment, contract terms, receipt, or reverse-charge triggers. The time of supply of services determines CGST/SGST liability and is generally the earlier of invoice date or receipt of payment when invoiced timely; otherwise the earlier of service completion or payment receipt, or the date the recipient records the service. For continuous supplies it is the contractual due date if ascertainable, otherwise each earlier of payment receipt or invoice issuance, or the event completion when payment is event-linked. Reverse-charge supplies use the earliest of service receipt, payment, invoice receipt or debit entry. Ceased supplies are deemed supplied when they cease; fallback rules use return filing date or tax payment date.