The sun is meant to be enjoyed – To lounge in, to play in, to bask in. But, you have to know the basics to make your time in the sun safer.
UVA and UVB rays from the sun can be damaging to the skin. If the proper sunscreen protection isn’t worn during sun exposure, this leads to aging, can cause unsightly skin damage and even skin cancers. While UVA rays go deeper into the skin causing damage to collagen leading to aging, wrinkles, and even skin cancers, UVB impacts superficially causing sunburn and skin cancers as well.
Sunscreens have ingredients that help in absorbing or scattering UV rays. These products are formulated to provide various levels of protection from UVA, UVB and Infrared radiation. 
The Sun Protection Factor (SPF) indicates how much longer an individual can be in the sun before being sunburnt when using a sun protection product than if he/she was not using it. It measures the protection only against UVB rays and not UVA. For instance, a fair skinned person who would normally start to feel the effect of sunburn after 10 minutes in the sun would receive 15 times the protection with an SPF 15 product i.e. 150 minutes of protection. If a person with darker skin takes longer to get sunburnt without protection, say 20 minutes, an SPF15 would give him/her 300 minutes of protection.
Most people do not apply enough sunscreen. An adult should use 2 tablespoons of sunscreen if they intend to protect the entire body while wearing a swimming costume. This should be applied 30 minutes prior to going outside. Apply 2 teaspoons full if only for the face, neck, and arms.
Some sunscreens may have an expiry date written on the box, however the active ingredients in the sunscreen deteriorate over time and this would happen faster if you leave it in direct sunlight, heat, or humidity. If it looks or smells differently or the texture is clumpy, then it might be expired.