sir,
According to Section 12 (2) of CGST Act, 2017 "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:
Provided that 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 amount shall, at the option of the said supplier, be the date of issue of invoice in respect of such excess amount."
This section is made applicable to IGST Act, 2017 vide Section 20 of IGST Act, 2017. Therefore time of supply of export goods is time issue of invoice.