Time of supply of services - earlier of invoice issuance or receipt of payment determines GST liability under forward charge. Where invoice is issued within the time prescribed under Section 31, time of supply is the earliest of: date of issue of invoice; date of receipt of payment to the extent it covers the supply; date of entry in the supplier's books; or date of credit in bank account. If invoice is not issued within the prescribed time, time of supply is the earliest of: date of provision of service; date of receipt of payment to the extent it covers the supply; date of entry in books; or date of bank credit. Failing these, recipient's book entry governs.