iCal events and time zones

I was in Melbourne last weekend. I had entered an event in iCal for the flight home while I was still in Adelaide, and I had not enabled time zone support in iCal. Of course, on arriving in Melbourne, the first thing I did was to update the system time zone to reflect local time—and consequently the departure time of my return flight was pushed back half an hour. I noticed this when I was looking at a hard copy of the flight details with about two hours to spare. I made the flight. And I've enabled time zone support in iCal.