If the sniff and visible-mold test isn’t palatable for your leftover food checks, the DaysAgo timer will count the number of days that have passed since you stuck that container or baby food jar in the fridge. It comes with either a magnet or suction cup attachment and a pack of two is about ten bucks. You can of course use it for all sorts of day-timing situations, watering plants, cleaning the kitty litter, anything up to 99 days. Here’s a Gadgeteer review from last year.