On my recent trip abroad, I didn't bother to adjust the date & time in my camera. One less thing to have to remember to do (and then undo). I knew when bringing the photos into Aperture, adjusting the time zone is a breeze. There are two ways to adjust the time zone.

1. Change the time zone during import
   
   When importing photos, open the Time Zone panel from the Import Settings pulldown. Specify the time zone you have set in the camera, what the actual time zone is where you shot the photos, and import. Viola! Time zones are updated.
    
2. Batch Change the time zone
   
   If you forget to change the time zone during import (like I just did :), don't worry. You can adjust the time zone of your photos at any time. Select the photos you want to change and select Metadata > Batch Change. There's a wealth of metadata you can adjust.
   
   For the time zone, the dialog window is pretty self explanatory. The semantics are the same as importing photos.

Strictly speaking, there's also a third way to adjust the time. You can select a set of photos and using Metadata > Adjust Date & Time. In the popup window, adjust the hour forward or backward as necessary. Aperture "does the right thing" and makes the corresponding adjustment across all the photos in your selection. Of course, with this method, you have to calculate the time zone offset yourself (if you're into that kind of thing :).