Talk about an old dog... this has been an often requested feature for well over a year. Sure, we were happy to get Date (Filename Friendly), et. al., but it sure would be great to get complete control over how dates are presented.
When I first tagged all my photos (about 2000 of them) I exported them all (using MC XML Export), transformed the MPL to convert the dates to all numbers (YYYYMMDD), deleted all the photos from the library, and then imported the transformed MPL. Not exactly easy, but it only took about twenty minutes (including the time to write the XSLT).
Boy... I'd really like to get a way to update the library with a partial MPL. Then at the least, I'd have complete control over creating my own "calculated" fields.
Here's a thought: How about a way to create our own calculated fields using some standard language? Sure, I'm a bit partial to XSLT or XQuery, but something SQL-like would be fine as well.