Posted launch time means that you should be prepared to actually launch at that time. If you are not prepared to launch at that time, you may be left behind. Therefore, you should plan accordingly and arrive at the launch site early.
Posted meeting time means we will be meeting at a given location at a particular time and then caravan to the actual launch site. If you arrive late at the meeting site, you may be left behind. Therefore, you should plan accordingly and arrive at the meeting site early.
