The premise, there's a seat in a coffee shop that allows you to travel to any other moment in time. The constraints - you can't leave the seat, and you only have as long as a cup of coffee stays warm.
The rules of the café are a bit silly, and repeated a few too many times, but the characters and the themes of the book are warm as a good cup of coffee, charming as a small out of the way café, and mostly very sweet in a way that coffee isn't. A time travel story that makes the simple point that what we really want when we fantasize about doing it is not a change to change the world, but to speak with someone.
Worth the short read.