activity.withStatus().setAttendees(attendees);
// TODO: is it needed to check if the attendees are changed?
*/
// check time constraints
Long duration = activity.withConstraints().withTime().getDuration();
if (duration != null) {
String start = activity.withStatus().getStart();
String end = activity.withStatus().getEnd();
if (start != null && end != null) {
DateTime startTime = new DateTime(start);