Martin
If she likes playing/gambling,you could suggest her to use a dice(or two,if she likes high "risks").You throw the dice(s),and the result would be the number of days she would have to wear it.
I could suggest you to make her accept to use a dice,without showing it to her and use a special dice,with 10 or 20 numbers,used by roleplayers,but I think she may not find this trick very fun...