1882?

However, Doc looked much older than when we last saw him in BTTF 3, so I'm assuming Doc, after raising his kids, decided to take a little holiday from the wife and revisit his stomping grounds in the Old West. He went back to 1882 because he knew that the Tannen gang hadn't come to town at that time and he could avoid all that drama. he just wanted to chill out a bit and enjoy tinkering with his new DeLorean, that he rebuilt after the last one got smashed. It was more convenient for time travel than the locomotive engine.