code: public RefreshInterval setRefreshInterval(Duration refreshInterval) {
  		RefreshInterval property = (refreshInterval == null) ? null : new RefreshInterval(refreshInterval);
  		setRefreshInterval(property);
  		return property;
  	}
docs: Sets the suggested minimum polling interval for checking for updates to
  the calendar data.
  @param refreshInterval the refresh interval or null to remove
  @return the property object that was created
  @see <a
  href="http://tools.ietf.org/html/draft-ietf-calext-extensions-01#page-7">draft-ietf-calext-extensions-01
  p.7</a>