ISSUE OF GST RENTAL INVOICE OF POST DATE

our client is having a rental property which was given to a big corporate company
they are asking us to submit the monthly rental invoice copy between 20-25th every month I.e before month end and invoice date should 1st day of next month

for example issuing invoice for the month of April 21 on 25th April mentioning date of invoice as 1st may

I want to know whether it is legally valid

thanks in advance