Well, I assist all challenges I see even if I am maxed out, cause somewhere there a mobster will be dying to get an extra shot to level up. but I must say way the limited time calendar xp goes down is good. then you know when to stop lol
I play around the same time each day, as I imagine most players do. So I don't see the need for this. But if some sort of timer is implemented like for the job helps so you know when the "new" day will start for you, that's fine with me.