Living in a military town, it seems to me the only people who appreciate punctuation, uh, punctuality are those in the military. I have done parties where I began an hour after the party started, ran a little over, and still had people coming in expecting to see the performance!
All of the above suggestions are good but, unfortunetely, there is no solution that works in every situation. Only experience will teach you which response works best for what situation.
Another aspect of this is being asked to stay longer but that's a subject for another thread.
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