This is a really cool idea; I have a several questions/ideas, mostly regarding mental time being stored (if that were ever to be added):
1.) Mental: Could you feasibly store, along with time, certain experiences / memories that you choose to forget?
2.) Mental: Could some objects only store physical-time and others only mental-time? Ex. Metal holds only physical time; wood holds only mental time; water holds both.
3.) Mental: If you stored away your entire life (and basically blinked out of existence), and if a child were to gain access to the stored time, would the child essentially be given a whole other life? This would also depend on what kind of time is in play. The child might appear to be 90 with the brain of a 2 year-old, or look the same but have the knowledge of the 90 year-old. Or he could age both ways.
4.) Physical: Would the mother of someone, who's life decreased almost back down to zero, have to go through childbirth again, or does the time-reverse only affect the one using it and not that of others significantly involved in the person's life?
I think that's it for me lol, this is a really awesome idea with a lot of potential for character development.