In the expression we leave at eight thirty for nine, what time is the departure going to be?
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There is possibly a little ambiguity here, since this type of expression is occasionally used to mean "we will plan to meet early in case there are delays or stragglers."
So it might for example be said "we will meet at 8:30 for a 9:00 start", meaning that it will acceptable to arrive at any point within the half-hour, though arriving early is encouraged.