Our recommendation is to buy for the season you're planting. But if you have seed left over, proper storage keeps it viable longer than most people expect.
 

To store unused seeds:
 

1. Seal them in an airtight container or plastic bag to protect against moisture and insects
2. Keep them somewhere cool, dark, and dry — away from sunlight and humidity
3. Skip the refrigerator; fluctuating moisture is harder on seed than temperature

Stored well, most seeds hold through the following growing season with excellent results. Our seeds typically lose less than 5% of their viability per year under ideal conditions.
 

Not sure if older seed is still good? A quick germination test will tell you:
 

Moisten a paper towel, place 10 seeds on one half, fold it over, and seal it in a plastic bag or airtight container. Keep it in a warm location appropriate for the variety, and check periodically over the seed's normal germination period. Count how many sprout — 8 of 10 means roughly 80% viability. If the rate is low, you can often compensate by sowing more thickly. Just keep in mind this measures viability under ideal conditions, so actual garden performance can vary with soil, moisture, and weather.