Mass Job Completion

What is the easiest way to mark all existing jobs completed up to a certain date. It can even be up to today.

Up until now we have never completed or closed jobs in the system going back to 2007. Someone in the past tried to set the AutoJob Completion Process but didn't associate it with the Product Groups correctly, so it never fired.

I'm looking for the least painful method of just mass flagging pre-existing jobs complete. We are on 8.03.408B SQL, and I really really don't want to update the SQL tables directly.