According to Eat By Date, if they are stored correctly, unopened cake mix and unopened icing can be used and eaten even after they are out of date. Eat By Date explains that, unopened, these pantry items each have a shelf life of about four months past the printed sell-by-date.
Eat By Date states that canned frosting can last for up to four months past the expiration date and still be used. Dry frosting lasts up to half a year. Dry cake mix can be stored, unopened, for about five months past its sell-by-date and still be usable. Unfortunately, once they are opened this can be reduced to only a few weeks.