wp_insert_post_data_example.php

<?php
/**
 * Plugin Name: wp_insert_post_data example
 * Plugin URI: https://gist.github.com/JPry/2e4b074a81d7c8234eb4
 * Description: Example of prevnting the modified date from changing.
 * Version: 1.0
 * Author: Jeremy Pry
 * Author URI: http://jeremypry.com/
 * License: GPL2
 */

// Prevent direct access to this file
if ( ! defined( 'ABSPATH' ) ) {
	die( "You can't do anything by accessing this file directly." );
}

add_filter( 'wp_insert_post_data', 'jpry_filter_insert_post_data', 10, 2 );
function jpry_filter_insert_post_data( $data, $postarr ) {
	// Only strip the modification time for Administrator role.
	if ( 'bil' === $data['post_type'] && current_user_can( 'administrator' ) ) {
		unset( $data['post_modified'] );
	}

	return $data;
}