Hi Jim,
I'm afraid you can't do that easily. Date / Time format customization is a weakness of MC. It's not too problematic for managing audio files but it's something that's missing whan managing photos.
The default "Date (filename friendly)" field always include the time and you can't access the date and time fields separately when creating calculated fields to setup your own format.
What I did was to use an other program (naborcorb cam2pc) to name my folder and files in the exact way I wanted (i.e. 2003-1225--Christmas party/2003-1225-001--Santa claus.jpg)
Then I imported those pics in MC,
Created a custom fields (named PIC Date),
and used the command Tools/Library Tools/Fill Properties from filenames with an appropriate template (i.e Directories: [PIC Date]--[Album]
It's the only way I found, other than manual entry, to enter a date in the format I always used (2003-1225) in MC library.
I hope that this will be improved in MC10 !
Hope this helps a bit,
Have a nice day,
Christian