More Info: Despite this requirement, there is a very high possibility that last summer's bottle of sunscreen won't be as helpful to you as one from right off the shelf. No matter what the SPF level or other additives the sunscreen should be applied thoroughly even on cloudy days, so leftover sunscreen shouldn't be a problem in the first place.
If old or leftover sunscreen must be used, you should be sure to exercise caution in doing so. Any sun damage can be permanent to your skin, but experimenting with the use of older sunscreen is best on days of short sun exposure times just to minimize the chances. Opting to use older sunscreen on a day long beach trip or venture to an amusement park might end up badly for you, and not just in the realm of sunburns.
Sunscreen loses potency after the expiration date on the bottle and after the FDA preset of three years, but buying new sunscreen is the safest option for proper skin care.