I change time zones a lot. It is very easy to mess up calendar entries: I am in New York, thinking about a dinner in London, and my calendar is still set to SF time.
One thing that helps is to actually have the desired time embedded in the appointment title text. Then, even if the screwed time zone stuff puts your dinner down as starting at 4am, you can still salvage the original data.
The problem is that Google Calendar will take the ‘7pm’ you stick into the text and use it to set the time and then delete it from the text. Helpful to set the time, unhelpful to delete the text.
I discovered a hack: you type the time as text in the ‘what:’ box (the text), and then, instead of typing the rest of the info in the ‘what:’ box, you click on ‘edit event details’. Because the 7pm is the only word in the ‘what:’, Google does not delete it, and it still sets the time. I then add the other info to the ‘what’ field in the calendar details. Note that I generally have to open the ‘edit event details’ anyway, since I want to note where the meeting is.