EvenList

void main() {
  List<Object> eventList = [
    Event(
      eventTime: DateTime(1961, 05, 12),
    ),
    [
      Event(
        eventTime: DateTime(1963, 05, 12),
      ),
      Event(
        eventTime: DateTime(1963, 05, 12),
      ),
      Event(
        eventTime: DateTime(1963, 05, 12),
      ),
    ],
    Event(
      eventTime: DateTime(1964, 05, 12),
    ),
  ];

  eventList = addNewEvent(
      Event(
        eventTime: DateTime(1964, 05, 12),
      ),
      eventList);

  print(eventList);
}

List<Object> addNewEvent(Event newEvent, List<Object> eventList) {
  List<Object> newList = eventList;
  if (eventList.any((element) {
    if (element is Event) {
      if (element.eventTime == newEvent.eventTime) {
        return true;
      }
      return false;
    } else {
      if ((element as List<Event>)[0].eventTime == newEvent.eventTime) {
        return true;
      }
      return false;
    }
  })) {
    for (var i = 0; i < eventList.length; i++) {
      if (eventList[i] is Event) {
        if ((eventList[i] as Event).eventTime == newEvent.eventTime) {
          newList[i] = [eventList[i], newEvent];
        }
      } else {
        if ((eventList[i] as List<Event>)[0].eventTime == newEvent.eventTime) {
          (newList[i] as List<Event>).add(newEvent);
        }
      }
    }
  } else {
    if (eventList.isNotEmpty) {
      for (var i = 0; i < eventList.length; i++) {
        if (eventList[i] is Event) {
          if ((eventList[i] as Event).eventTime.compareTo(newEvent.eventTime) ==
              1) {
            newList.insert(i, newEvent);
          }
        }
      }
    } else {
      newList.add(newEvent);
    }
  }

  return newList;
}

class Event {
  DateTime eventTime;

  Event({required this.eventTime});
}