function milliseconds( $dp = null ){
	 	if( $this->is_running() ){
		 	$now = microtime( false );
		}
		else {
			$now = $this->stopped;
		}
		$started = self::parse_microtime( $this->started );
		$stopped = self::parse_microtime( $now );
		$ms = $stopped - $started;
		if( ! is_null($dp) ){
			$mult = pow( 10, $dp );
			$ms = round( $mult * $ms ) / $mult;
		}
		return $ms;
	 }