Modified date instead of Published date

Hello, my website is using twenty fourteen theme and I have made some changes to CSS. Everything works good except the date part. I want to show modified date on top of the post instead of published date. I found this code

<?php

/**
 * Print HTML with meta information for the current post-date/time and author.
 * @since Twenty Fourteen 1.0
 * @example http://wp.me/p1lTu0-bsQ
 */

if ( ! function_exists( 'twentyfourteen_posted_on' ) ) :

function twentyfourteen_posted_on() {
	if ( is_sticky() && is_home() && ! is_paged() ) {
		echo '<span class="featured-post">' . __( 'Sticky', 'twentyfourteen' ) . '</span>';
	}

	printf( '<span class="entry-date"><a href="%1$s" rel="bookmark"><time class="entry-date" datetime="%2$s">%3$s</time></a></span> <span class="byline"><span class="author vcard"><a class="url fn n" href="%4$s" rel="author">%5$s</a></span></span>',
		esc_url( get_permalink() ),
		esc_attr( get_the_modified_date( 'c' ) ),
		esc_html( get_the_modified_date() ),
		esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
		get_the_author()
	);
}
endif; ?>

When I place this code in functions.php, it crashes my website. Is there any idea where should I place the code? Or is there any other way of showing last modified date?