According to Mayo Clinic, sunblock does have an expiration date, and it is usually found somewhere on the bottle. Not all sunblock has an expiration date listed, but typically it is still acceptable to use it three years after first usage.
If the expiration date is not listed on a bottle of sunblock, then it is a good idea to write the date of purchase on it and use it for a maximum of three years after that date. Furthermore, a customer service number should be listed on the label, and this number can be called to find out if a bottle of sunblock has already expired. According to About.com, sunblock that lasts for at least three years does not require an expiration date, while those that expire within three years after first use are legally bound to provide an expiration date on the bottle of sunblock.