
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
static FunctionPtr create() { return std::make_shared<FunctionConvertFromString>(); }
    String getName() const override
    {
        return name;
    }
    bool isVariadic() const override { return true; }
    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & ) const override { return true; }
    size_t getNumberOfArguments() const override { return 0; }
    bool useDefaultImplementationForConstants() const override { return true; }
    bool canBeExecutedOnDefaultArguments() const override { return false; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override { return {1}; }
    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
    {
        DataTypePtr res;
        if (isDateTime64<Name, ToDataType>(arguments))
        {
            validateFunctionArgumentTypes(*this, arguments,
                FunctionArgumentDescriptors{{"string", &isStringOrFixedString<IDataType>, nullptr, "String or FixedString"}},
                
                FunctionArgumentDescriptors{
                    {"precision", &isUInt8<IDataType>, isColumnConst, "const UInt8"},
                    {"timezone", &isStringOrFixedString<IDataType>, isColumnConst, "const String or FixedString"},
                });
            UInt64 scale = to_datetime64 ? DataTypeDateTime64::default_scale : 0;
            if (arguments.size() > 1)
                scale = extractToDecimalScale(arguments[1]);
            const auto timezone = extractTimeZoneNameFromFunctionArguments(arguments, 2, 0);
            res = scale == 0 ? res = std::make_shared<DataTypeDateTime>(timezone) : std::make_shared<DataTypeDateTime64>(scale, timezone);
        }