In the expression we leave at eight thirty for nine, what time is the departure going to be?
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.
It depends where you are going and how you are going there, it also depends which departure you are talking about. If you are leaving your house to get to a railway station (or bus stop or rendezvous point) from which the actual journey will start you will depart from your house at 7:30, take about 20 minutes to get to the station then the train (or bus or convoy) will depart at 8:00.
The form is also used for performance times. If you are in a band or and the instruction is to be at a gig venue at 7:00 for 8:00 you are expected to arrive at 7:00 so that PA can be set up, instruments tuned, sound check done and perhaps makeup and costume sorted out ready for an 8:00 start.
It's also used for dinner parties, but that is a little less exact. If a dinner invitation says "7:30 for 8:00" you should arrive closer to 7:30 than 8:00 so that you can mingle, you can, possibly, drink an apertif and your host is sure that the guests have arrived in good time. Everyone can then be seated around table and the cook (who might be your host) can serve the meal when it is ready.