Right now I just time it off of autosave ticks - every 15 minutes....
I've been using the autosaves as a timer too! Especially now that mine autosaves every 1'40"! (Mine used to be every 5 minutes)! Are the more frequent autosaves a feature of Rebellion?? (To check the time after an autosave notice, hit menu, load game, switch to autosave list, and look at time on the top/last autosave, then just exit back out to the game).
But I too would really like an on-screen timer. Like the replay clock/timer.