protected function valueToDatetime ($fmt, $value)
    {
        // ISO format YYYY-MM-DD HH:MM:SS as input
        if ($value == "0000-00-00 00:00:00") {
            return "";
        }
        if ($fmt == null) {
            $fmt = DATETIME_FORMAT;
        }
        return $this->valueToDate($fmt, $value);
    }