I don't know if I've been lucky in the past or what, but I've planted seeds from after-season sales that were 2-3 years old and the plants came up just fine. I would imagine that some of the seeds may have deteriorated and won't sprout, but the corn, zuchinni and cukes in my garden right now are from seed that is at least two years old.
Store the seeds in a paperbag, in the veg crisper drawer in the 'fridge. It's called stratifying, and we do it all the time at my nursery & garden center. Long term, put them in the freezer, as it simulates a winter freeze and does no harm.