All I can remember is that a young boy enters a science lab and finds a time machine with a capsule, and travels through time and ends up in the past in a forest, where he stumbles across an old man. The old man looks at the control panel and sees what looks to me like a car odometer, but is slowly going backwards like a countdown. It's been programmed to return automatically. I think the boy goes on an adventure or something, and barely makes it back in time when the time capsule returns to the base part in its own time. The movie was made in the 70's or early 80's, I do believe. Does this ring a bell with anyone else?