random posts in one category 15 posts back

Hi,

I’m looking for a way in a custom query to fetch 5 random posts among the last 15 posts in one given category. Is there a way to do this?

Right now this is what I’ve got:

<?php global $post;
	$args = array(
        'orderby' => 'rand',
	'numberposts' => 5,
	'category' => 5
	);
	$myposts = get_posts( $args );
	foreach( $myposts as $post ) : setup_postdata($post); ?>

Which gives me 5 random posts that are in category 5, but with no limit back in time/posts numbers.

Thanks!