Sync Google Calendars for Free
My calendar chaos!
I often work for multiple clients at once on the basis of outcomes delivered. Recently I was helping a government agency to adopt OKRs. It was a high-touch engagement involving the design and facilitation of dozens of workshops for hundreds of people across 30+ teams, writing internal guidance, building and deploying some custom tooling, and nurturing an internal group of OKR Ambassadors over more than half a year. To simplify scheduling and collaboration the client gave me an account on their Google Workspace.
Sounds great, right? Using their Google Workspace account ensured data protection and compliance and made me more effective. I couldn’t have taken on such a large and complex engagement without it but managing two separate Google Calendars quickly became a logistical nightmare. Between juggling my personal life and time with other clients, I’d occasionally double-book myself—or worse, miss appointments entirely.
I quickly realised I needed a better way to sync both calendars. I looked briefly at some commercial solutions and then I remembered the power & simplicity of Google Apps Script (GAS).
I wondered if I could build something simple myself or with help from generative AI!
Getting help from GenAI
I opened ChatGPT and described what I wanted via this prompt:
Immediately, I had a mostly working script!
I kept chatting to do a bit more refinement; I wanted to disable superfluous reminders on the HOLD events and access the free/busy flag on events directly via the Calendar Event API. It turns out there’s no way to access this directly but I got the script working well enough within 15 minutes. It syncs in both directions by loading events from both calendars by adding a “HOLD” placeholder in one calendar for accepted events in the other.
It does this automatically every hour, automatically cleaning up the placeholders if the original event gets canceled, deleted, or declined and prevents unnecessary reminders from being created on “HOLD” events.
I was really impressed with the cleanliness of the code, the reusable functions, and the code for creating and removing the trigger.
You can read my entire conversation with ChatGPT.
And the cost? Nothing. Zero. Only the time it took me to chat with the AI and set it up.
It’s been happily keeping both calendars in harmony for 6 months with zero issues and zero intervention on my part.
How you can use it
Are you also juggling multiple Google calendars across workspaces that need to be kept in sync? Give the script below a whirl and let me know how you get on!
- Open Google Apps Script and create a new project.
- Copy and paste the script (below) into the editor.
- Replace the placeholder calendar IDs with your own.
- Run it once (you’ll need to authorize it) to test it.
- Call the
createTrigger()function to set up automatic syncing.
Happy AI Coding!
This script has taken a huge administrative weight off my shoulders and the experience of being able to build simple apps like this is a game changer and adds a tremendous value to my already favourite productivity suite. I can’t wait to build more.
I hope it helps you, too. I’d love to hear from you how you’re using it as well as other experiences you’ve had with GAS and ChatGPT to build simple utilities like this.
1// Sync two calendars by creating a 'HOLD' event in the corresponding calendar (or deleting as appropriate)
2// Runs every X minutes between A-B hours (see createTrigger function)
3
4var DEBUG_MODE = false; // Set to true for debug mode (only logs events without modifying them)
5
6function syncCalendars() {
7 // Replace with your Calendar IDs
8 var calendarId1 = 'XXX@XXXXXX.com';
9 var calendarId2 = 'XXX.XXX@XXX.XXXX.com';
10
11 var holdEventName = 'xHOLDx'; // Change this to whatever name you prefer
12 var daysAheadToSync = 20;
13
14 var calendar1 = CalendarApp.getCalendarById(calendarId1);
15 var calendar2 = CalendarApp.getCalendarById(calendarId2);
16
17 var now = new Date();
18 var startTime = new Date(now.getFullYear(), now.getMonth(), now.getDate());
19 var endTime = new Date(startTime);
20 endTime.setDate(endTime.getDate() + daysAheadToSync);
21
22 // Load cached processed events
23 var properties = PropertiesService.getScriptProperties();
24 var processedEvents = JSON.parse(properties.getProperty('processedEvents') || '{}');
25
26 syncFromSourceToTarget(calendar1, calendar2, startTime, endTime, holdEventName, processedEvents);
27 syncFromSourceToTarget(calendar2, calendar1, startTime, endTime, holdEventName, processedEvents);
28
29 // Save updated processed events
30 properties.setProperty('processedEvents', JSON.stringify(processedEvents));
31}
32
33function syncFromSourceToTarget(sourceCalendar, targetCalendar, startTime, endTime, holdEventName, processedEvents) {
34 var sourceEvents = sourceCalendar.getEvents(startTime, endTime);
35 var targetEvents = targetCalendar.getEvents(startTime, endTime, { search: holdEventName });
36
37 var deletionDelayHours = 2; // Delay deletion by 2 hours
38
39 // Remove HOLD events in target calendar if the source event no longer exists
40 targetEvents.forEach(function (targetEvent) {
41 var relatedEvent = sourceEvents.find(function (event) {
42 return event.getStartTime().getTime() === targetEvent.getStartTime().getTime() &&
43 event.getEndTime().getTime() === targetEvent.getEndTime().getTime();
44 });
45
46 var now = new Date();
47 var eventStart = targetEvent.getStartTime();
48 if (!relatedEvent && (eventStart - now) / 3600000 > deletionDelayHours) {
49 Logger.log('[DEBUG] Would delete: ' + targetEvent.getTitle() + ' starting at ' + targetEvent.getStartTime());
50 if (!DEBUG_MODE) {
51 retryWithBackoff(function () {
52 targetEvent.deleteEvent();
53 }, 3);
54 }
55 }
56 });
57
58 // Add HOLD events in target calendar for accepted events in source calendar
59 sourceEvents.forEach(function (sourceEvent) {
60 var eventTitle = sourceEvent.getTitle();
61
62 // Ignore HOLD events
63 if (eventTitle !== holdEventName) {
64 var isAllDay = sourceEvent.isAllDayEvent();
65 var containsDay = /day/i.test(eventTitle);
66
67
68 // Skip only all-day events that end in "day"
69 if (!(isAllDay && containsDay)) {
70 var myStatus = sourceEvent.getMyStatus(); // Get user's response
71 if (myStatus === CalendarApp.GuestStatus.YES || myStatus === CalendarApp.GuestStatus.OWNER ) { // Only sync if the event is accepted
72 var relatedEvent = targetEvents.find(function (event) {
73 return event.getStartTime().getTime() === sourceEvent.getStartTime().getTime() &&
74 event.getEndTime().getTime() === sourceEvent.getEndTime().getTime();
75 });
76
77 if (!relatedEvent && !processedEvents[sourceEvent.getId()]) {
78 if (!DEBUG_MODE) {
79 retryWithBackoff(function () {
80 var newEvent = targetCalendar.createEvent(holdEventName, sourceEvent.getStartTime(), sourceEvent.getEndTime());
81 newEvent.removeAllReminders(); // Remove reminders only at creation
82 }, 3);
83
84 processedEvents[sourceEvent.getId()] = true; // Mark event as processed
85 }else {
86 Logger.log('[DEBUG] Would create: ' + eventTitle + ' starting at ' + sourceEvent.getStartTime());
87 }
88 }else {
89 if (DEBUG_MODE) {
90 Logger.log('[DEBUG] Not syncing ' + eventTitle + 'related title: ' + relatedEvent.getTitle());
91 }
92 }
93 }
94 } else {
95 if (DEBUG_MODE) {
96 Logger.log('[DEBUG] Skipping all-day event ending with "day": ' + eventTitle);
97 }
98 }
99 }
100 });
101}
102
103function createTrigger() {
104 ScriptApp.newTrigger('syncCalendars')
105 .timeBased()
106 .everyHours(2) // Runs every 2 hours instead of every 20 minutes
107 .create();
108}
109
110function deleteTriggers() {
111 var triggers = ScriptApp.getProjectTriggers();
112 triggers.forEach(function (trigger) {
113 ScriptApp.deleteTrigger(trigger);
114 });
115}
116
117// Helper function to handle retries with exponential backoff
118function retryWithBackoff(fn, retries) {
119 for (var i = 0; i < retries; i++) {
120 try {
121 return fn();
122 } catch (e) {
123 Logger.log('Error: ' + e.message + ' (Retrying in ' + (i + 1) + 's)');
124 Utilities.sleep((i + 1) * 1000); // Wait before retrying
125 }
126 }
127}
About Peter
With more than 25 years of experience, I use OKRs, agile practices, and more to help teams maximise clarity, delivery, and joy.
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