Round.TRIP

Man returns to earth. Planet has no memory of sending him on voyage.

He was supposed to travel to nearest planet with expedition. Disaster on way and ship heads back.

He has AI he ironically name Dave. obvious 2001 ref. Controls ship and companion.

Turns out it took much longer to return to earth. Earth was fighting war, destroyed most life. AI waited orbiting solar system until civilization built back up. Several thousand years. More maybe?