Adding last updated code does not work on this theme?

Hi, I found the perfect widget for what i'm trying to do on this blog post: https://ariel-lim.com/blog/show-last-modified-date-wordpress-website/. But it wont work on my theme? What gived, heres the code. any thoughts? Thanks a million in advanced! :)

// Published & Modified Date
function be_published_modified_date() {
	$date = get_the_date( 'U' );
	$updated = get_the_modified_date( 'U' );

	$output = '<span class="entry-date"><span class="label">Published on</span> ' . get_the_date( 'F j, Y' ) . '</span>';
	if( $updated > ( $date + MONTH_IN_SECONDS ) )
		$output .= ' <span class="entry-date-modified"><span class="label">• Last updated</span> ' . get_the_modified_date( 'F j, Y' ) . '</span>';

	return $output;
}
add_shortcode( 'be_published_modified_date', 'be_published_modified_date' );