Due Date Flag/indicator change when due date is on or before today

I have a due date of an item and a field "today's date". When that due date is on or before today's date, I have a business rule that sets an due date indicator to YES. My issue is I am having to open the form to get the business rule to run. I need to automatically change when due date is today's date. I can do a one time update to all the past records but I'd like it to be automatic for any future ones so the form does not have to be opened to change. Thanks for any help. (by the way, it wouldn't let me select version 9 but that is what version we are on)