Outstanding invoice total

What is the best way to take a table of Account Payable information and total all the invoice amounts that are still open where the net_due_date is older than today. 

Essentially, what is the total amount of invoices that are past the net_due_date. 

I was starting down the path of If(net_due_date > today, sum(invoice total field) as a measure. I think I may be missing a row by row trigger. Thanks for the help, I'm still learning!