Retrieves the date on which the post was last modified.

function get_the_modified_date( $format = '', $post = null ) {
	$post = get_post( $post );

	if ( ! $post ) {
		// For backward compatibility, failures go through the filter below.
		$the_time = false;
	} else {
		$_format = ! empty( $format ) ? $format : get_option( 'date_format' );

		$the_time = get_post_modified_time( $_format, false, $post, true );
	}

	/**
	 * Filters the date a post was last modified.
	 *
	 * @since 2.1.0
	 * @since 4.6.0 Added the `$post` parameter.
	 *
	 * @param string|int|false $the_time The formatted date or false if no post is found.
	 * @param string           $format   PHP date format.
	 * @param WP_Post|null     $post     WP_Post object or null if no post is found.
	 */
	return apply_filters( 'get_the_modified_date', $the_time, $format, $post );
}