All the seeds packaged for last years growing season have a 'sell by' date of the end of last year on them. The questions is "How much does it really matter?" Well sometimes it matters a lot, and other times not at all. A seed is alive inside, and as long as it remains alive it can germinate and grow. These seeds will produce plants just as healthy as if it had been their first year! There is no difference in the plants from old seeds, just the germination rate.
3 different factors come into play when using old seeds: age, type and storage.
Age: all seeds will be viable for a year...many for 2 years, some as long as 6 years....or maybe even more.
Type: Things like peppers and corn don't store well for long periods, but lettuce and cucumber last the longest. I've used 3 year old bean seeds with no problem.
Storage: Cool and dark is optimal.
So, what does this mean? Well, did the store keep the seeds well? I'm guessing they were in a box in the warehouse all winter. That sounds like cool and dark to me. They are only 1 year old. They are only $.10 each. (now granted these weren't expensive seeds to begin with, but I digress) My point here is that even if you need to germinate a few extra seeds to get your desired amount of plants, you're saving money in the long run. Chances are you don't use a whole seed packet for every type of plant anyway. For things like pumpkins, cucumbers or peppers I only want 6 plants. Flowers I plant more of, but still often have seeds left over. Which brings me back to the 3 factors I mentioned before. Your left over seeds from last year or the year before follow those rules also. So store your seeds in a cool, dark place and try to use them up in 2 years. Ideally, let a few plants go to seed and save some of your own...but feel free to search the bargain bin for new varieties. You'll still get great results from last years seeds.