`sf.apply` not working with dates

Using danfo-node, with node 16...

`timestamp` is a column created from `new Date()` instance

``` javascript
const sfHourly = df.column('timestamp').apply((date) => {
    const dt = new Date(date);
    dt.setMinutes(0);
    
    return dt;
});
```

This code throws the following  message `TypeError: arr.slice is not a function`