protected function applyMask($unit)
    {
        $cron = \explode(' ', $this->expression);
        $mask = ['0', '0', '1', '1', '*', '*'];
        $fpos = $this->fieldsPosition[$unit] - 1;

        \array_splice($cron, 0, $fpos, \array_slice($mask, 0, $fpos));

        return $this->cron(\implode(' ', $cron));
    }