Time of supply of services determines GST liability by invoice issuance or receipt of payment, with special reverse charge and voucher rules. Time of supply of services is the earlier of invoice issuance (or required invoice date) and receipt of payment; payment counts when entered in books or credited to bank. Reverse charge supplies use the earlier of payment or the date immediately after sixty days from invoice, with a fallback to the recipient's books and a special rule for associated enterprises located outside India. Voucher supplies use the issue date if supply is identifiable, otherwise the redemption date. If none of these rules determine time of supply, use the periodic return filing date or, otherwise, the date on which CGST/SGST is paid.