
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
data_to.resize(size * (strlen("YYYY-MM-DD") + 1));
            else if constexpr (std::is_same_v<FromDataType, DataTypeDateTime>)
                data_to.resize(size * (strlen("YYYY-MM-DD hh:mm:ss") + 1));
            else if constexpr (std::is_same_v<FromDataType, DataTypeDateTime64>)
                data_to.resize(size * (strlen("YYYY-MM-DD hh:mm:ss.") + col_from->getScale() + 1));
            else
                data_to.resize(size * 3);   
            offsets_to.resize(size);
            WriteBufferFromVector<ColumnString::Chars> write_buffer(data_to);
            if (null_map)
            {
                for (size_t i = 0; i < size; ++i)
                {
                    bool is_ok = FormatImpl<FromDataType>::template execute<bool>(vec_from[i], write_buffer, &type, time_zone);
                    null_map->getData()[i] |= !is_ok;
                    writeChar(0, write_buffer);
                    offsets_to[i] = write_buffer.count();
                }
            }
            else
            {
                for (size_t i = 0; i < size; ++i)
                {
                    FormatImpl<FromDataType>::template execute<void>(vec_from[i], write_buffer, &type, time_zone);
                    writeChar(0, write_buffer);
                    offsets_to[i] = write_buffer.count();
                }
            }
            write_buffer.finalize();
            if (null_map)