The Outer Limits S02E12 - Inconstant Moon

When a lonely physics professor realizes the sun is going to burn out, he knows that he and humanity have only a few hours to live. In that time, Stan resolves to make up for lost years. He courts and proposes to Leslie, a woman he has known for two years but never dated. Leslie, although puzzled, finds herself more and more entranced by Stan until she finds out the reason for Stan's sudden attention.