We won't be able to add DATE on a paid invoice, director5.
Instead, we can click the 1 payment made (amount) hyperlink to see the payment date.
Aside from that, you can also print the invoice payment. It will show you the payment date, payment method, and other details it.
I'll let our Product Development Team know about your idea of adding the payment date in the Invoice window. They'll evaluate and put it into consideration for future product updates.
You can always reach out to us if you need anything else.