I would be aiming for 9.15 but would give myself the half hour window if I wasn't sure of what the traffic would be like. If I knew roughly what it was like around that time I would happily state 9.15 by itself.
There is only one person who I allow to be a little bit late if we are going on elsewhere (such as cinema) as she has an idiot dog, anyone else gets my wrath and surpringly enough they are never late for me again but continue to be late for other people, so it really is a choice they make.