For other people, distinguishing between a date and a not-date is difficult, if not impossible. Thus, those who are on a not-date are often confused by other people that they are dating or vice versa.
In the base metaphor, a not-date is the equivalent of sitting in the bleachers and watching the game.
Ryan: "So then Claire and I went to dinner and then watched a movie at my place."
Jake: "All right! So did you hit that?"
Ryan: "Hell no. It was a not-date."
(at a restaurant)
Waiter: "Okay, you're total's going to be $23.44."
Jimmy: "Can you make those two separate orders?"
Waiter: "Sorry about that, I thought that you were on a date."
Jimmy: "No, we're friends. This is a not-date."