Add Timestamp to WordPress Child Theme Stylesheet to invalidate cache

gistfile1.txt

function add_timestamp_to_childtheme_stylesheet() {
    wp_dequeue_style( 'style' );
    wp_deregister_style( 'style' );
    wp_enqueue_style('style', get_stylesheet_uri().'?'.filemtime(get_stylesheet_directory().'/style.css'), array(), null);
}
add_action( 'wp_print_styles', 'add_timestamp_to_childtheme_stylesheet' );