New report: payment history per open item (when was an invoice paid?)
(available from V7.7.2245)

With this new report you can easily check when an invoice has been paid:
Reports => Open items => Payment history per open item

From where should be calculated? (due date or voucher date)
Optional restrictions: Account from/to and due date from/to

You will get a list of open items incl. display when the invoice was paid and how many days have passed since the voucher date or due date.