It should work...I just don't think it propagates the custom calculated field to the list when attaching your custom content type.

Steps Taken:

 1. Created a custom content type based on the Event content type.
 2. Added a calculated field = [Start Time].  
 3. Added custom content type to Calendar List.  
 4. Caveat...Had to manually add the site column to the list.   
 5. Added an event.