I'm sure you have a few bottles of sunscreen lying around from last Summer. Will they still protect your skin even though they're a year old?
Sunscreen can last up to three years if it was stored in a cool, dry place, with the cap tightly closed. If it wasn't, then it should be thrown out. Extreme heat can cause the active ingredients to deteriorate, so they'll be less effective at protecting you from the sun's harmful rays. If you're not sure, it's best to pick up a new bottle.
You can also check the sunscreen's expiration date. If it hasn't expired yet, check the lotion's consistency. If it is runny, gloppy, or smells weird, it has probably gone bad, and you should spring for some new sunscreen. It's really not worth saving money if your skin's health is compromised, right?