The following steps have allowed me to successfully edit the start and stop times of a variety of songs:
* Right-click on the track you want to edit and select "Get Info".
* In the pop-out window select the "Options" tab.
* Enter the "Stop-time" or "Start-time" you want to use and make sure the checkbox to the right is selected.
* Now hit "Ok" in the lower right corner to close the pop-up. (This is where one would logically stop but logic does not exist in this excercise)
* Right-click on the track you editted again and select "Create AAC Version" - you will see a new grayed out copy below your original track - after a few seconds the grayed out track will "ping" and you're editted start or stop time are applied