Merge multiple google calendars (copy events from many sources into one calendar)

merge_calendars.gs

// Reference: https://developers.google.com/apps-script/reference/calendar/calendar-app

function sync(sources, target) {
  var start = new Date((new Date()).getTime() - 10 * 3600000 * 24);
  var end = new Date((new Date()).getTime() + 120 * 3600000 * 24);
  var targetCalendar = CalendarApp.getCalendarById(target);
  if (!targetCalendar) {
    return;
  }
  var targetEvents = targetCalendar.getEvents(start, end);
  
  for (var sourceIndex=0; sourceIndex<sources.length; sourceIndex++) {
    var source = sources[sourceIndex];
    
    // Clean deleted events
    var clean = function (targetEvents) {
      for (var i=0; i<targetEvents.length;) {
        if ((targetEvents[i].getTag('original') || '').indexOf(source) == 0) {
          targetEvents[i].deleteEvent();
          targetEvents.splice(i, 1);
        } else {
          i++;
        }
      }
    };
    
    var sourceCalendar = CalendarApp.getCalendarById(source);
    if (!sourceCalendar) {
      clean(targetEvents);
      return;
    }

    // clean();
    
    // Create or update events...
    var sourceEvents = sourceCalendar.getEvents(start, end);
    for (var sourceEventIndex=0; sourceEventIndex<sourceEvents.length; sourceEventIndex++) {
      var sourceEvent = sourceEvents[sourceEventIndex];
      var original = sourceCalendar.getId() + "::" + sourceEvent.getId();
      var targetEvent = null;
      for (var i=0; i<targetEvents.length; i++) {
        if (targetEvents[i].getTag('original') == original) {
          targetEvent = targetEvents[i];
          targetEvents.splice(i, 1);
          break;
        }
      }
      if (!targetEvent) {
        if (sourceEvent.isAllDayEvent()) {
          targetEvent = targetCalendar.createAllDayEvent(sourceEvent.getTitle(), sourceEvent.getStartTime(), sourceEvent.getEndTime());
        } else {
          targetEvent = targetCalendar.createEvent(sourceEvent.getTitle(), sourceEvent.getStartTime(), sourceEvent.getEndTime());
        }
        targetEvent.setTag('original', original);
      }
      if (sourceEvent.getTitle() != targetEvent.getTitle()) {
        targetEvent.setTitle(sourceEvent.getTitle());
      }
      if (sourceEvent.isAllDayEvent() != targetEvent.isAllDayEvent() ||
          sourceEvent.getStartTime().valueOf() != targetEvent.getStartTime().valueOf() ||
          sourceEvent.getEndTime().valueOf() != targetEvent.getEndTime().valueOf()) {
        if (sourceEvent.isAllDayEvent()) {
          targetEvent.setAllDayDate(sourceEvent.getStartTime());
        } else {
          targetEvent.setTime(sourceEvent.getStartTime(), sourceEvent.getEndTime())
        }
      }
      if (sourceEvent.getDescription() != targetEvent.getDescription()) {
        targetEvent.setDescription(sourceEvent.getDescription());
      }
    };
    
    // Remove deleted events
    clean(targetEvents);
  };
  
  // Delete orphans...
  for (var i=0; i<targetEvents.length; i++) {
    targetEvents[i].deleteEvent();
  }
}

function syncTeam() {
  sync(
    [
      "source1@group.calendar.google.com",  // Calendar id
      "source2@group.calendar.google.com",
      "source3@group.calendar.google.com",
    ],
    "target@group.calendar.google.com"  // Use a new calendar as target
  );
}

Create a trigger on the AppScript editor to run this function every hour.