For starters, while some bottles do have a printed expiration date, not all of them do. If you find a questionable bottle of sunscreen with nary an expiration date in sight, you’d be better off throwing it in the garbage and buying a new bottle. Sure, you can risk it, but that means you’re risking getting a burn, which can increase your chances of developing skin cancer.
Also, certain storage conditions can alter a sunscreen’s effectiveness. Keeping a spare bottle in your car, for example, sounds like a good idea — after all, it’ll always be with you for impromptu walks on the beach or an unplanned visit to the zoo. But excessive heat (or direct exposure to the sun) can dramatically change how well the sunscreen actually works.