public function getFilename($file_name, $rotation_index = 0)
    {
        if ($rotation_index===0) return $file_name;
        $rotation_date = date(
            $this->options['date_format'],
            time() - ($this->options['period_duration'] * $rotation_index)
        );
        return strtr(
            sprintf($this->options['filename_mask'], $file_name),
            array(
                $this->options['filename_date_tag'] => $rotation_date,
                $this->options['filename_iterator_tag'] => $rotation_index,
            )
        );
    }