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

It is confused by a comma - should be easy to fix.
return on_error("Cannot read DateTime: unexpected number of decimal digits after year: " + toString(num_digits), ErrorCodes::CANNOT_PARSE_DATETIME);
                    
                    if (!day_of_month && checkChar(delimiter_after_year, in))
                    {
                        num_digits = readDigits(digits, sizeof(digits), in);
                        if (num_digits == 2)
                            readDecimalNumber<2>(day_of_month, digits);
                        else if (num_digits == 1)
                            readDecimalNumber<1>(day_of_month, digits);
                        else if (delimiter_after_year == ' ')
                            continue;
                        else
                            return on_error("Cannot read DateTime: unexpected number of decimal digits after year and month: " + toString(num_digits), ErrorCodes::CANNOT_PARSE_DATETIME);
                    }
                }
            }
            else if (num_digits == 2 || num_digits == 1)
            {
                
                
                
                
                
                
                
                
                
                
                UInt8 hour_or_day_of_month_or_month = 0;
                if (num_digits == 2)