Yeah, sorry my bad.
I realise now that I had in my head a vision of your setup that was close to this and that you already had date fields for the state change, and you wanted to report on the delta between two time stamps.
Not sure why I thought that! Serves me right for posting after a long day at work 
Your comment about looking into the backend should have clued me in.
Anyway, yes, I guess that you need to do something on the entity side of things (either action buttons or formulas) to get at least some of the way there. Sorry!