Worked for Me.

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.

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.