Why do you throw milk out when it expires? It can make you sick. You should treat your soft contact lenses the same way. After they’re manufactured, lenses are sterilized and sealed before being put on the market. If too much time passes they risk becoming contaminated, even if left in the package. This could cause serious eye problems.
The expiry date on the lens package is generally written in yyyy/mm format. For example, if the package reads 2015/03, the contents are sterile and safe to wear until March 2015. This date is usually four years from the time of manufacture. Of course this is different from your prescription itself, which in most cases has to be renewed once a year.
Don’t risk your eyes—throw out old lenses!