Seed sprouts quick in warm soil. 12 weeks before frost--that is unworkable. You are right, Smallaxe.
Actually, in my experience you can seed grass seed in the fall in spite of frost--it won't hurt a thing. Germination and growth are just very slow (or zero) during the part of the day that is below about 45. Growth resumes in the spring. Germination and growth began in my town about when the soil temp reached 55 degrees f--which is about the date of "greenup", or April 15 in my town. The early sprouts were a quarter inch tall, about April 20 in 2014.