Time of supply of goods determines GST liability based on invoice issuance, payment receipt, reverse charge and voucher rules. The liability to pay CGST/SGST arises at the time of supply, ordinarily the earlier of invoice issuance (or last invoice date) and payment receipt, with payment treated as the earlier of book entry or bank credit. Under reverse charge the earliest of goods receipt, payment by recipient, or thirty days after invoice determines time, or failing these the recipient's books control. For vouchers the time is issue if identifiable, otherwise redemption. If none apply, time of supply is the return filing date or the date CGST/SGST is paid.