He never worked for the post office..

The first time he visits Louie, they talk about how Sarah threw him out because he gave up his academic career to become a postman. And he says, “After Vietnam I just never wanted to think anymore.” Meaning he was still an academic when he got drafted; he dropped his academic career AFTER Vietnam. Also, his Army buddies call him “Professor” in Vietnam.

He also knows things in his “dreams” that he couldn’t possibly have known in 1971 (the date of the battle.)

He knows what fashions/music etc. would be like in the mid-70s. At the party, Jezzie dances to “Lady Marmalade” which was a big hit in 1975. Jezzie also feeds him a Hungryman tv dinner—those didn’t hit the market until ‘73. Also he had no way of knowing about The Ladder drug while he was being operated on in the mobile army hospital.

He was being shown visions of his future by a higher power. That he would lose his family and career, and be tormented by government agents for talking about The Ladder.