Accounting Core allows as long as the accounting period is opened to submit invoices for not only the current period but also past and previous periods. Thus as long as the August period is opened for both AP and GL, entering invoices that need to be entered in August can be entered and posted. This way they reflect the correct period.
I have created a Case for this - as I want to make sure you have the information needed to assure these invoices are entered as needed.
Case number : CAS-278740-K8H7P5