ConcatenateGoogleCalendars.gs

// From Resources > Advanced Google Services, enable Calendar API,
// and follow the link "Google Developers Console" and enable Calendar API there, too.
//
// Original: https://github.com/kdotsaito/GAS/blob/master/calender_merge.gs
// Calendar ID to concatenate to.
var outCal_id = "@group.calendar.google.com";
// Calendar IDs to concatenate.
var inCal_ids = [
    "@group.calendar.google.com",
    "@group.calendar.google.com",
    "@group.calendar.google.com",
    "@group.calendar.google.com" ];

function main(){

    var months = 4;
    var date_start = new Date();
    var date_end = new Date;
    date_end.setTime(date_start.getTime() + months*30*24*60*60*1000);

    Logger.log("Retrieving output calendar data.");
    var outCal = CalendarApp.getCalendarById(outCal_id);
    var old_Future_Ev = []
    old_Future_Ev = old_Future_Ev.concat(CalendarApp.getCalendarById(outCal_id).getEvents(date_start,date_end));

    Logger.log("Retrieving input calendar data.");
    var new_Future_Ev = [];
    var calendar_title = [];
    for(var i = 0; i < inCal_ids.length; i++){
            var cal_name = CalendarApp.getCalendarById(inCal_ids[i]).getName()
            var events_to_add = CalendarApp.getCalendarById(inCal_ids[i]).getEvents(date_start,date_end);
            new_Future_Ev = new_Future_Ev.concat(events_to_add);
        for(var j = 0; j < events_to_add.length; j++){
            calendar_title.push(cal_name);
        }
    }

    var remove_flag = [];
    for(var j = 0; j < old_Future_Ev.length; j++){ remove_flag[j] = true; }  // init
    var add_flag = [];
    for(var i = 0; i < new_Future_Ev.length; i++){ add_flag[i] = true; }  // init

    Logger.log("Looking for new/updated events.");
    for(var i = 0; i < new_Future_Ev.length; i++){
        for(var j = 0; j < old_Future_Ev.length; j++){
            if( isSameEvent(calendar_title[i], new_Future_Ev[i], old_Future_Ev[j]) ){
                remove_flag[j] = false;
                add_flag[i] = false;
                break;
            }
        }
    }

    for(var j = 0; j < old_Future_Ev.length; j++){
        if(remove_flag[j] == true){
            Utilities.sleep(500);  // Avoid deleting events too quickly.
            Logger.log("Deleting event: " + old_Future_Ev[j].getTitle());
            old_Future_Ev[j].deleteEvent();
        }
    }

    for(var i = 0; i < new_Future_Ev.length; i++){
        if(add_flag[i] == true){
            Utilities.sleep(500);  // Avoid adding events too quickly.
            Logger.log("Adding event: "+ new_Future_Ev[i].getTitle());
            if(new_Future_Ev[i].isAllDayEvent()){
                createAllDaysEvent(createTitle(calendar_title[i], new_Future_Ev[i].getTitle()),
                new_Future_Ev[i].getAllDayStartDate(),
                new_Future_Ev[i].getAllDayEndDate(),
                new_Future_Ev[i].getDescription(),
                new_Future_Ev[i].getLocation());
            }else{
                outCal.createEvent(createTitle(calendar_title[i], new_Future_Ev[i].getTitle()),
                new_Future_Ev[i].getStartTime(),
                new_Future_Ev[i].getEndTime(),
                {description:new_Future_Ev[i].getDescription(),
                 location:new_Future_Ev[i].getLocation()});
            }
        }
    }
}

function createAllDaysEvent(summary, start, end, description, location) {
  var calendarId = outCal_id;
  var event = {
    summary: summary,
    location: location,
    description: description,
    start: {
      date: add9Hours(start).toISOString().substring(0, 10)
    },
    end: {
      date: add9Hours(end).toISOString().substring(0, 10)
    }
  };
  event = Calendar.Events.insert(event, calendarId);
}

function add9Hours(d){return new Date(d.valueOf()+9*60*60*1000)}

function createTitle(calendar_title, event_title){
    return "<" + calendar_title + ">" + event_title;
}

function isSameEvent(event1_calendar_title, event1, event2){
    if (! (createTitle(event1_calendar_title, event1.getTitle()) === event2.getTitle()) ) {return false;}
    else if (! (event1.getStartTime().getTime() === event2.getStartTime().getTime()) ) {return false;}
    else if (! (event1.getEndTime().getTime() === event2.getEndTime().getTime()) ) {return false;}
    else if (! (event1.getDescription === event2.getDescription) ) {return false;}
    else if (! (event1.getLocation() === event2.getLocation()) ) {return false;}
    else {return true;}
}