Hey Jan,
have you checked the API documentation
According to the documentation, the view_date parameter will only return the format of the registered user submitting the call - so this will only work if you have access to the user account. I did not find any solution to get the format for any registered user unfortunately.
If a request specifies the parameter restapi.response_style with the value view, the response will include attributes that relate the elements to the standard web interface. The attributes include
| Attribute | Description |
|---|---|
| view_href | For elements that are associated with a page in the standard web interface, this specifies the URL for that page. |
| view_date | Included in date/time elements. This provides a date string formatted as expected by the Lithium user associated with the call. |
| view_time | Included in date/time elements. This provides a time string formatted as expected by the Lithium user associated with the call. |
