This screen deals with the ‘Period’ option whether the actual dates are only set at short notice (or where you want to run a 4/4/5 week rotation). In these instances, you need to set the date prior to each run.
Enter the new Period Date and click the button with the Plus icon to add it to the list. The list shows all the previous Period Dates
Invoiced shows Yes if the invoices have been run against a particular date.