8 episode mini-series on Hulu. Premise is there is a mysterious time portal in the closet of a small diner, always sends the person back to the same instant in 1960. The diner owner used it secretly trying to stop the assassination of JFK 3 years later by trying to figure out who really did it. However the original guy eventually gets cancer and has to pass the task to a new, younger guy. That's basically the start of the story. Also the way the time porta works, any changes you make in the past carry over to the future, but anytime you go through the portal again it erases any changes you made last time. Finally- whether you spend a minute or a year in the past, when you come back only 2 minutes have passed in the current time.
The thought is preventing JFK's assassination would more or less prevent the Vietnam War and make the world a better place. I assume the diner owner lost someone close in the war but it wasn't made clear in the 1st episode.
