invoice date

You have to consider the time of supply for deciding that in which month outward supply details needs to be disclosed..

Time of supply for services is as follows:

 a. If the tax invoice is raised with in 30 days from the date of provision of service then : Invoice date or date of receipt of payment which ever is earlier..

b.  a. If the tax invoice is not raised with in 30 days from the date of provision of service then: date of provision of service or date of receipt of payment which ever is earlier...

So based on the above provision if the time of supply is aug then you have to show in Aug month GST return. If TOS is in Sept then show it in GST return of sept month