Shouldn’t be too hard. I remember the film pretty well, but have no idea of any title or actor names. I saw it on TV mid-70s. looked like it was made early to mid-60s.
Its about a group of scientists who build a view screen to look into the future or the past. First they use it to look 100,000 years ahead. It comes on for a few seconds then shorts out. Then they reset it to look only a few hundred years ahead, and while doing that the female scientist thinks she sees something strange move in the lab. Then, while viewing the less-distant future a non-scientist in the lab (janitor or something?) discovers that its not just a view screen but a portal, and he jumps thru it to the future. They all go thru to try and get him back and of course the machine shorts out again leaving them all stranded in the future.
Amazingly to the scientists, the short-term future is a Wells-ian dystopia with a mutant class and humans trying to leave via spaceship. Most of the film takes place here. The future humans don’t have room to take the scientists from the past with them, so they let them try and build another time viewer-portal-thingy to get back to the past. They succeed (just as the future humans’ rocket is blown up by the mutants) and they go back to right before they left.
But something isn’t right because their past selves seem frozen in time. They’re at the point in the beginning of the film when they had their original machine set to 100,000 years in the future so they decide to walk thru the portal then. This distant future seems peaceful and pleasant. Just then they switch to the past scientists and the scene where the woman thought she saw something move by (it was their future selves, get it?) and then the whole story starts to repeat over & over faster & faster, then the credits roll.
I’m sure that’s more than enough. I think I’m making it sound cooler than it actually was…