In terms of Section 12(4) of the CGST Act, 2017, time of supply of vouchers shall be the earliest of the following:
(a) date of issue of voucher, if the supply is identifiable at that point; or
(b) date of redemption of voucher, in all other cases. Eg: Mr. A buys vouchers from Lifestyle of worth ` 1,000/- for a shirt dated August 01, 2017. Mr. A gifts such vouchers to Mr. B who redeems such vouchers with Amazon India on August 31, 2018. – Time of supply is the date of issue of vouchers viz., August 01, 2017.
(ICAI FAQ PUBLICATIONS 06-09-2017 TIME FOR SUPPLY OF GOODS: FAQ NO. 7)