Filtering the_excerpt doesn't

I have inserted this into my functions.php file as a test case as I am trying to modify the excerpt, and it appears to have no effect at all:

function md_add_dateline( $content ) {
	return "HELLO";
}
add_filter( 'the_excerpt ', 'md_add_dateline');

Is there something obvious I’m missing? The ultimate goal is to insert the post date as an old-school newspaper dateline, so that it looks like this:

May 7, 2012 – Lorum ipsum…

Any tips? It must be something obvious right?

ETA: I have a space after ‘the_excerpt ‘ which is the source of the problem. Fixed.