State and federal seed laws require seed companies print dates on the seed packages. Each state has different requirement, ranging from 9 to 18 month. West coast states usually have longer dates than eastern states, probably having to do with when the laws were set, western states were still the "frontier" states so they were allowing the seeds to be sold in a longer time period. Seeds don't "expire" Per Se after the 9-18 month after the printed dates. Vitality is used to measures how viable seeds are, the longer the time has passed, the poorer the vitality of the seeds. Different seeds maintain their vitality different. Beans, cucumbers and peas will last 5-6 years, while onions, lettuce and carrots probably only 1-2 years, that is from when the seeds were harvested, not what's printed on the seed package. Commercial seed sellers can sell their seeds as long as the germination meets the state requirement. Vitality is not a required element. So to answer your question, you can use past date seeds, just be aware that some may not come up fast. And avoid using old onion/leak seeds, because they have the least storage life.
Good luck and happy gardening!