the_modified_date( string $format = '', string $before = '', string $after = '', bool $display = true )

function the_modified_date( $d = '', $before = '', $after = '', $echo = true ) {
	$the_modified_date = $before . get_the_modified_date($d) . $after;

	/**
	 * Filters the date a post was last modified for display.
	 *
	 * @since WP-2.1.0
	 *
	 * @param string $the_modified_date The last modified date.
	 * @param string $d                 PHP date format. Defaults to 'date_format' option
	 *                                  if not specified.
	 * @param string $before            HTML output before the date.
	 * @param string $after             HTML output after the date.
	 */
	$the_modified_date = apply_filters( 'the_modified_date', $the_modified_date, $d, $before, $after );

	if ( $echo )
		echo $the_modified_date;
	else
		return $the_modified_date;

}