If someone is giving you a half hour of time, I'd bet on them being a terminally late person, and mentally see it as 9:30, and expect them at 10am. But if you know the person usually tries to be punctual, then 9:30 at the latest.
It depends on why they've given you a half hour window of arrival time - if it's to do with unreliable public transport, for example, then I'd cut them slack, and get ready for 9:30, but not expect them to arrive on the dot.