Unless indicated by an expiration date, the FDA requires that all sunscreens be stable and at their original strength for at least three years.
You can use the sunscreen that you bought last summer, but keep in mind that if you are using the appropriate amount, a bottle of sunscreen should not last very long. About 1 ounce of sunscreen, enough to fill a shot glass, is considered to be the amount needed to cover the exposed areas of the body properly.