21.2:

```
milovidov-desktop :) SELECT CAST(1111111111.222 AS DateTime64(3))

SELECT cast(1111111111.222, 'DateTime64(3)')

Query id: d10c8623-5c74-4a38-b535-a99f6af4d28d

┌─cast(1111111111.222, 'DateTime64(3)')─┐
│               2005-03-18 04:58:31.222 │
└───────────────────────────────────────┘
```

master:

```
milovidov-desktop :) SELECT CAST(1111111111.222 AS DateTime64(3))

SELECT cast(1111111111.222, 'DateTime64(3)')

Query id: cb4af967-a4ae-4df5-89ba-234d68675f93

┌─cast(1111111111.222, 'DateTime64(3)')─┐
│               2005-03-18 04:58:31.000 │
└───────────────────────────────────────┘
```
if (isDateTime64<Name, ToDataType>(arguments))
            {
                timezone_arg_position += 1;
                scale = static_cast<UInt32>(arguments[1].column->get64(0));
                if (to_datetime64 || scale != 0) 
                    return std::make_shared<DataTypeDateTime64>(scale,
                        extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
                return std::make_shared<DataTypeDateTime>(extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
            }
            if constexpr (std::is_same_v<ToDataType, DataTypeDateTime>)
                return std::make_shared<DataTypeDateTime>(extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
            else if constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
                throw Exception("Unexpected branch in code of conversion function: it is a bug.", ErrorCodes::LOGICAL_ERROR);
            else
                return std::make_shared<ToDataType>();
        }
    }
    
    
    
    bool useDefaultImplementationForNulls() const override { return checked_return_type; }
    bool useDefaultImplementationForConstants() const override { return true; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override
    {
        if constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
            return {2};
        return {1};
    }
    bool canBeExecutedOnDefaultArguments() const override { return false; }
    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count) const override