shelf life of grass seed
Some of the seeds will be viable. You can test the seeds by placing a few in a damp folded paper towel. Keep in a warm spot for a few days (don't let it dry out) and observe what percentage germinate. Then, when you use the seed, you'll know how heavily to sow the seed to compensate for the non-viable seed. For example, if half of the seed germinates and is alive, then sow the seeds twice as thickly.
Late summer/early fall is the best time to sow seed.
For more about seeding, read through this good fact sheet: http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102%20La... It gives normal seeding rates.