add_action( 'genesis_entry_content', 'sk_display_new', 9 );
function sk_display_new() {

    // if the current page is not the Posts page (as set at Settings > Reading), go away
    if ( ! is_home() )  {
        return;
    }

    global $post;

    if ( strtotime( $post->post_date ) > strtotime( '-7 days' ) ) {
        echo '<span style="color:#863136;font-size:16px;text-align:center;"><b>NY! (Siste 7 dager)</b></span>';
    }

}