
select toDateTime64(-86400 ,9,'UTC') ==  1969-12-31 00:00:00.000000000

select toDateTime64(-86400.00 ,9,'UTC') ==  1970-01-01 00:00:00.000000000 


However, I expect both outputs to be same. 

Version affected : 22.8
if (!read_buffer.eof())
                {
                    if (result_type)
                        throwExceptionForIncompletelyParsedValue(read_buffer, *result_type);
                    else
                        throw Exception(ErrorCodes::CANNOT_PARSE_TEXT,
                            "Cannot parse string to column {}. Expected eof", column_to.getName());
                }
            }
        }
        else
            throw Exception(ErrorCodes::ILLEGAL_COLUMN,
                "Illegal column {} of first argument of conversion function from string",
                column_from.getName());
    }
};
template <>
struct ConvertImpl<DataTypeString, DataTypeUInt32, NameToUnixTimestamp, ConvertDefaultBehaviorTag>
    : ConvertImpl<DataTypeString, DataTypeDateTime, NameToUnixTimestamp, ConvertDefaultBehaviorTag> {};
template <>
struct ConvertImpl<DataTypeString, DataTypeUInt32, NameToUnixTimestamp, ConvertReturnNullOnErrorTag>
    : ConvertImpl<DataTypeString, DataTypeDateTime, NameToUnixTimestamp, ConvertReturnNullOnErrorTag> {};
/** If types are identical, just take reference to column.
  */
template <typename T, typename Name>
requires (!T::is_parametric)
struct ConvertImpl<T, T, Name, ConvertDefaultBehaviorTag>
{
    template <typename Additions = void *>
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t ,