What if a coupon has no expiration date?

In the U.S., many foods are not required to carry an expiration date. I would not purchase an item that is known to have a short shelf life without either a production date, a sell-by, an expiration or a best-by. Most diet sodas will have a best-by date (usually only a few months) since the artificial sweetener can break down and leave you with an awful tasting concoction. Regular soda can last significantly longer.

If you have a specific product in mind, contact the manufacturer to get information on recommended shelf life and to tell them you don't appreciate their not having a use-by date on the package.