How about a different approach? Since it sounds like you are using getResources can you use the publish and unpublish dates?
I am using this...
-- Upload files to a Dropbox account.
-- Dynamic Image Generator
-- Google URL Shortener
-- Uses google chart api to make QR codes.
-- Update your twitter status on publish.
So you're using MIGX, not getResources? It would be useful if you posted your full getPage call. All the answers, mine included, have assumed you were using getResources in your getPage call. FYI Articles uses getResources to list resources and I'm not sure you'll be well served with Articles.
A MODx Fanatic
I would use a custom-table and a MIGXdb - TV or create a MIGXdb - CMP