Add an “Updated Recently” Label to WordPress Posts

/**
 * Add an "Updated Recently" label to recently modified posts
 */
function custom_add_updated_recently_label( $title, $post_id ) {

    if ( is_admin() || get_post_type( $post_id ) !== 'post' ) {
        return $title;
    }

    $modified_time = get_post_modified_time( 'U', true, $post_id );
    $days_ago     = ( time() - $modified_time ) / DAY_IN_SECONDS;

    if ( $days_ago <= 7 ) {
        $title .= ' <span class="updated-recently">Updated Recently</span>';
    }

    return $title;
}

add_filter( 'the_title', 'custom_add_updated_recently_label', 10, 2 );