You should declare this advance or enter invoice details only when GST invoice is issued at the time of supply.
Time of supply means the point in time when goods/services are considered supplied’. When the seller knows the ‘time’, it helps him identify due date for payment of taxes.
As per Section 12 of the GST Act, The time of supply of goods shall be the earlier of the following dates, namely:—
(a) the date of issue of invoice by the supplier or the last date on which he is required, under sub-section (1) of section 31, 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.
Thus GST on Supply of Goods was payable at time of receipt of advance, if the amount is received before issue of invoice
GST will not be payable at time of receipt of advance in case of outward supply of goods by registered persons having turnover less than 1.50 Crore during last FY, w.e.f. 13th October 2017. However GST on advance is payable if advance received during period 01/07/2017 to 12/10/2017. Not applicable for Composition scheme.