When you need to be on time for something, it means that you already made the vital mistake of telling someone that you would be there at a certain time and/or openly agreeing to that, which cannot be done, sometimes. Sometimes, things go wrong, and you can't quite make it over there, and yet they get real mad at you for no good reason. They don't understand why you didn't show up, and they couldn't really care less why you weren't on time.