Save and display the last modified post date in WordPress

functions.php

<?php
/**
 * Adds a timestamp in the first blog option.
 * @return void
 */
function juiz_update_last_edit_date() {
	if ( is_multisite() ) {
		add_blog_option( 1, 'last_post_modified_timestamp', time() );
	} else {
		add_option( 'last_post_modified_timestamp', time() );
	}
}
add_action('save_post', 'juiz_update_last_edit_date' );

/**
 * Echo the formatted date based on the $format param and the last modified post date.
 * @param  (string) $format The formatted date based on datetime format.
 * @see https://www.php.net/manual/en/datetime.format.php
 * @return (string) The formatted date.
 */
function juiz_last_edit_date( $format ) {
	if ( is_multisite() ) {
		$timestamp = get_blog_option( 1, 'last_post_modified_timestamp', 0 );
	} else {
		$timestamp = get_option( 'last_post_modified_timestamp', 0 );
	}
	
	echo date( $format, $timestamp );
}

your-template.php

<?php
  /**
   * Use this wherever you want to display the date
   */
  juiz_last_edit_date('m.d.Y');

?>