We recently did a shoot in a location in the MDT time zone (UTC -6) and imported the files into Aperture. Upon returning to our home location in the PDT time zone (UTC -7) and beginning editing we've discovered a bug with editing using an external editor.
just to make sure - how did you import? Did you specify the camera's timezone and the actual time zone on import? Otherwise Aperture will assume the camera's Time Zone to have been the local time zone your computer is set to.
The time zone adjustment is buggy for some Canon Cameras. What brand of camera do you use? And what is your external editor?
I am asking all this, because I never had a problem with the time zone and external edits - only when using my Canon Camera