I like this story. I am pretty sure it was based on the Winston Niles Rumfoord character, in the Kurt Vonnegut book "The Sirens of Titan." That would make this story sort of a cousin or sibling of Watchmen, because the Dr. Manhattan character was also based on Winston Niles Rumfoord.

Anyway, this is an interesting story, but it has sort of a complicated style. I think it's worth spending time figuring it out. The basic idea is the a guy who had trained as an astronaut is hired for a time travel experiment, and he does travel in time, but on his way back to his home time something goes very wrong, which ends up resulting in a pretty large nuclear war. Any nuclear war would be "pretty large," of course, but my point is that this war did a hell of a lot of damage, but ultimately did not extinguish humanity altogether. In the story, you get glimpses of people gradually rebuilding, over many centuries, sort of like in A Canticle for Leibowitz.

Anyway, I could say more about it, but here, just read it, and if anybody wants to talk about it, I'm game.
https://www.baen.com/Chapters/9781625791542/9781625791542___2.htm