How does Aperture know the exact date and time in the metadata though? It's not GMT based, so you can change it around as you see fit.
Maybe someday cameras will have built in GPS that will not only geotag the photos, but also set the time based on GMT so this will all be easy. But for now, time is relative. Some people try and set the camera's time to local time wherever they go, and don't adjust in Aperture. Other people NEVER set the time, and then use Aperture to offset from GMT to correct. And if you travel and adjust time in various locations, you may have to do things differently based on where you are.
When you import your images to Aperture, the program will assume the camera to be set to the time zone your system is set to, unless you specify a time zone correction during the import session.
You can specify the time zone the camera was set to and the "actual time": the time zone that you want the date on the images to show.
In case you forgot to set this information during the import session, you can use the batch change command to correct this later:
Metadata > Batch Change ; then enable "Time Adjustment" and set it to "Adjust Time Zone".
Set the "Camera's Time" to PDT, and the "Actual Time" as well. Take care to set the "Add Metada from" menu to "None".
P.S. If your camera is a Canon, this fix might not work. For some Canon cameras the time zone correction still is buggy and shows no result.
You make some very valid points. I never really thought about it that way. I just assumed that even though you can change the date and time at random, Aperture could still tap into it and read it for when you changed your file names. File naming obviously isn't based off of that though.
Thanks for the input.