Let’s imagine you’ve got packets of seeds leftover from last year and are considering using them again. But it’s not worth it if they won’t grow, I know.
BUT! Seeds are usually viable for 3-5 years. An easy test to check them: take 5-10 seeds, put them in a moist paper towel for a couple of days and approximate your germination percentage. For example, if 5 seeds of the 10 germinate, you have 50% germination and should probably plant some extra seeds to get the number of plants you want.
The date on the package is usually what season they are sold–so if the package says 2012, the seeds were harvested the previous season for sale in 2012 and should be just fine until 2015.