Time of supply in GST determines tax timing: invoice or payment date governs liability, with special procedures for suppliers. Time of supply under GST fixes when tax liability arises: for goods supplied by normal registered persons (excluding composition dealers) the time of supply is the invoice issuance date (or last date to issue the invoice); otherwise the earlier of invoice date (or last date to issue) and date of receipt of payment. For services the earliest of invoice date (if issued within the prescribed period) or date of receipt of payment applies, with alternate rules if invoice is late. Special reverse-charge, voucher, cross-border associated enterprise and residual provisions further determine timing, and additions to value by interest/penalty are timed to receipt.