Solution: You can set the correct timezones in Planapple's itinerary. See this article for details on setting timezones.
Explanation: TripIt does not give Planapple the timezone for imported items. Instead, TripIt provides all your itinerary information in the universal "UTC" timezone. Planapple tries to convert flight times to the correct local timezone based on the airport codes. But if Planapple doesn't recognize the airport code, or for items in your TripIt itinerary that don't have an associated airport code, Planapple has to guess at a timezone to use: either your home timezone (which you can change in your profile), or the UTC timezone from TripIt.