Wordpress - enqueue styles with dynamic version

functions.php

function theme_enqueue_styles() {
    $stylesheet_url = get_stylesheet_directory_uri() . '/style.css';
    $stylesheet_date_modified = date( 'YmdHi', filemtime( $stylesheet_url ) )
    wp_enqueue_style( 'theme-slug-style', $stylesheet_url, array( 'iron-master' ), $stylesheet_date_modified, 'all' );
}
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );