time of supply

then what should it mean

As per Section 2(118) of the CGST Act 2017 :

 "VOUCHER" means an instrument where there is an obligation to accept it as consideration or part consideration for a supply of goods or services or both and where the goods or services or both to be supplied or the identities of their potential suppliers are either indicated on the instrument itself or in related documentation, including the terms and conditions of use of such instrument.

In addition to above defination , the voucher means that instrument which be accepted as consideration wholly or partly for a supply. Thus voucher is deemed as asset for receipent. its a instrument which can used for effecting another Inward supply.

if at the time of voucher issue it is not consideration then why we consider time of supply at the time voucher issue