`startDate` and `endDate` should be type Date

Currently I have to do this to work with this component:
```ts
  const onDateChange = ({ startDate }: { startDate: number }) =>
    (selectedDate = new Date(startDate))
```
whereas a more proper solution would be to bind to something like this which `cally` is doing:
```tsx
onfocusday={(e: CustomEvent<Date>) => (selectedDate = e.detail)}
```

Is this too tricky or just a preference?