Time of supply of goods :
The time of supply of goods shall be the earlier of the following dates [section 12(2) of CGST Act]-
(a) the date of issue of invoice by the supplier or the last date on which he is required, under section 31(1) of CGST Act, to issue the invoice with respect to the supply or
(b) the date on which the supplier receives the payment with respect to the supply.
GST if advance upto Rs 1,000 received can be paid at the time of issue of invoice – Where the supplier of taxable goods receives an amount up to one thousand rupees in excess of the amount indicated in the tax invoice, the time of supply to the extent of such excess shall, at the option of the said supplier, be the date of issue of invoice – proviso to section 12(1) of CGST Act.
Thus, if advance upto Rs 1,000 is received, GST is not payable at that stage. In that case, GST will be payable when invoice is raised.