Before the Coffee Gets Cold by T. Kawaguchi

 In a quirky little Japanese cafe, there is a table with one seat that allows you to travel to the past or the future--but you must return to the present before the coffee in front of you gets cold. And nothing you say or do during your travels can change the present. Except these brief trips through time do have a profound influence on the lives of those who make the journey. This is a short but very thoughtful story.