There was a movie I saw a long time ago that seemed very similar to Back to the Future. It had two teenage children, I think a boy and a girl and the old guy inventor of the time machine. They travel around time for some reason. The reason this movie stands out for me is that at the end, they end up in the distant future. Someone gives the inventor guy some medicine. This medicine allows him to live forever, but only if he takes the medicine daily. If not, he will die. This forces him to stay in the future, but the children are able to make it back. I think the movie also ends with a happy note, like maybe the children get a note from the doctor in the present to the let them know that eventually he will be okay, not sure about that last part, but yeah. This has been bugging me for years and I really wanted to figure it out to recommend to people because if I remember correctly, it wasn't a bad movie. If anyone can help me out, I would appreciate it. Thank you.