`select parseDateTimeBestEffort('01/12/2017, 18:31:44')`

It is confused by a comma - should be easy to fix.
time_t & res,
    ReadBuffer & in,
    const DateLUTImpl & local_time_zone,
    const DateLUTImpl & utc_time_zone,
    DateTimeSubsecondPart * fractional)
{
    auto on_error = [](const std::string & message [[maybe_unused]], int code [[maybe_unused]])
    {
        if constexpr (std::is_same_v<ReturnType, void>)
            throw ParsingException(message, code);
        else
            return false;
    };
    res = 0;
    UInt16 year = 0;
    UInt8 month = 0;
    UInt8 day_of_month = 0;
    UInt8 hour = 0;
    UInt8 minute = 0;
    UInt8 second = 0;