Wow! I wondered if I have made it on to Mumsnet daily yet...
Wildbhoysmama - you're my favourite.
Exactly the below.
*The habitually late are just that though, habitually late. Those posters who accuse them of narcism are projecting their outrage at being kept waiting; "how dare they do this to me". Me, I, myself, aka the most important person, ergo the person who should never be kept waiting. The habitually late keep everyone waiting. It's not about you. For some reason or other they are poor time-keepers. Do them a favour, either cut them out of your lives so they can find more empathetic friends or accept this facet of their personality. If they can't change for themselves, they ain't gonna change for you. Accept it or move on.
As for the habitually early, now interestingly enough this is a perfect example of narcism. Turning up early to a dinner invite at someone's house is the height of rudeness as it puts undue pressure on the host. As a recruiter, I would sooner hire the occasionally late person over the person who is obtuse enough to think that sitting in reception for 20 minutes is acceptable. This smacks of self-importance and failure to recognise social norms.*
Sleepohowimissyou, mummyof3kids and skislope thank you for finally bringing a bit of empathy, rationale and understanding to this thread. Nice we haven't all got our judgy pants pulled up to our necks! Interesting points also about people who are self important enough to think being early and expecting everyone else to be too is any more acceptable than being late.
How self centred indeed to think you're so special that friend must not offend you in anyway what so ever, they must always walk on egg shells. I too can just label people as selfish. You must really think you're more important than your friend. 
I stop reading any posts that mentions "I have a friend like you..." - they are not like me, you don't know me. Your anecdote does not apply to me and is not the situation I described. You're angry at them, not me.
As for the poster who can control when your baby takes a shit, wow that is an amazing skill. I'm glad for you their bum knows you're leaving and behaves itself.
I do admit I've been a bit petulant in some of my replies, but then suddenly being called a twat seemed to have triggered that reaction.
Though thinking about the genuine suggestions to give a window instead, say 12-12.30, there will always be someone who will arrive after 12.30? What then? That person could have arrived 12.05/ 12.10 because we said 12, better than arriving 12.35!
Going to work/ interview, you have to be professional, under pressure to perform. Meeting a friend for a social should be relaxing and have no kick back whatever happens. Strict schedules are not for fun times.