The opening scene states that the events of the first murder take place in 1975. Sgt. Vince Korsak says that he was 18 years old and dating the victim's sister, Mary, at that time. The character has stated multiple times throughout the show that he served with the Marine Corps in Vietnam. The last American troops left Vietnam in March of 1973, with most having left by (at the latest) the end of 1972.
Vince Korsak states the victim was 77 years old and was the father of his old girlfriend. Bruce McGill, who plays Sgt. Korsak was 66 years old at the time. Unless there was a huge age difference between Vince and his girlfriend, the ages don't work.