How to set "date added" metadata in Mac OS X 10.7 Lion

The Finder in Mac OS X 10.7 Lion shows a new piece of file metadata, "Date Added," which tracks the date an item was added to a folder. After upgrading to 10.7, none of the items in my ~/Downloads folder have "Date Added" values. I'd like to set all the empty "Date Added" values to match the "Date Modified" values, but I can't figure out how to set the "Date Added" attribute to a specific value. My first guess was this: xattr -w com.apple.metadata:kMDItemDateAdded "2012-02-19 16:34:47 +0000" myfile But that doesn't seem to work (though it doesn't report an error either).