ui.input_date() doesn't support years <1000 and >9999

Error:
```
RECV: {"method":"update","data":{"x:shiny.date":"900-02-09"}}
_send_error_response: time data '900-02-09' does not match format '%Y-%m-%d'
```

Ideally, it would be great to have BC dates as well (year<0). 