REST API and date field format
To set a Date Field via REST in Drupal 8, you can do this:

// Build the JSON payload.
var node = {
  "type":[{"target_id":"article"}],
  "title":[{"value":"Hello World"}],
  "field_start":[{"value":"2017-07-21T18:11:11"}]
};

// Then POST the payload to Drupal...
$http.post('https://example.com/entity/node', node);