Time of supply rules determine when GST liability arises and whether invoice or payment fixes the taxable event.

Time of supply fixes tax liability at the earlier of invoice issuance (or required invoice date) and receipt of payment, with supply measured to the extent covered by invoice or payment and payment deemed the earlier of book entry or bank credit. For supplies under reverse charge, time of supply is earliest of receipt of goods, recipient's payment entry or bank debit, or the date following thirty days after supplier's invoice, with recipient's books as fallback. Voucher supplies use issue date if identifiable or redemption date otherwise; fallback rules tie time of supply to return filing date or tax payment date. Additions by interest, fee or penalty are taxable when received.

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