If you can’t remember the last time your bare skin saw the light of day, think twice before applying the first bottle of SPF you find stashed in your bathroom and ask yourself the important question: Does sunscreen expire? Similar to how mascara dries out and gets clumpy, sunscreen also deteriorates over time. “The ingredients in SPF can break down, losing their effectiveness to filter or block ultraviolet light,” says dermatologist Allison Arthur, MD, a board-certified dermatologist in Orlando, Florida. Not only can using an old, expired formula be ineffective, but it also can result in a nasty sunburn. Here’s everything else you need to know about how sunscreen expires so you can stay safe in the sun. Just like a carton of milk, skincare products have expiration dates. Before applying, double-check what the packaging says to ensure it’s still fresh. But beware: Some bottles may not have one listed. “If you buy a sunscreen that lacks an expiration date, it’s helpful to write the purchase date on the product so you know how long you’ve had it,” advises Dr. Arthur. Forgot to label it? A general rule of thumb is that if it’s over a year old, it’s time to replace it. T...