public function currentDate($type = 'date')
    {
        if (! in_array($type, ['date', 'timestamp'])) {
            throw new InvalidArgumentException('Type for currentDate operator must be date or timestamp.');
        }

        $this->requiresCurrentField();
        $this->newObj['$currentDate'][$this->currentField]['$type'] = $type;
        return $this;
    }