
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
{
        ColumnPtr result_column;
        if constexpr (to_decimal)
            result_column = executeInternal<ToDataType>(arguments, result_type, input_rows_count,
                assert_cast<const ToDataType &>(*removeNullable(result_type)).getScale());
        else
        {
            if (isDateTime64<Name, ToDataType>(arguments))
            {
                UInt64 scale = to_datetime64 ? DataTypeDateTime64::default_scale : 0;
                if (arguments.size() > 1)
                    scale = extractToDecimalScale(arguments[1]);
                if (scale == 0)
                    result_column = executeInternal<DataTypeDateTime>(arguments, result_type, input_rows_count);
                else
                {
                    result_column = executeInternal<DataTypeDateTime64>(arguments, result_type, input_rows_count, static_cast<UInt32>(scale));
                }
            }
            else
            {
                result_column = executeInternal<ToDataType>(arguments, result_type, input_rows_count);