Time of supply for spectrum usage services: GST payable when payment is due or made under reverse charge. Clarification addresses the time of supply for GST on government spectrum allocation where the supplier is DoT and the recipient is the telecom operator subject to reverse charge. Spectrum usage provided under deferred instalment contracts qualifies as a continuous supply; for reverse-charge supplies time of supply is the earlier of payment (or bank debit) or sixty days after issue of supplier's invoice. Frequency Assignment Letters are bid-acceptance documents, and tax invoices must be issued on or before each contractually ascertainable payment due date. GST is payable when upfront payment is made or due, or for deferred instalments as and when each instalment is due or paid.