Prepopulate year and month fields with default values from underlying site folderpath

I have an article content type that has Year and Month fields representing the Year and Month the article will be issued

The article content items will always be created under a site folder path like //Sites/MySite/2011/11. I would like the new article Year field defaults to year from the folder path (2011 in this case) and Month field defaults to month from the folder path (11 in this case).

Any idea how to implement this?

Thanks, Hua