Google Calendar Synchronization

Synchronize Google Calendar events between two or more calendars.

Made with Google Apps Script, related to Google Calendar Corrections.

Installation

1. Backup all Google Calendars to be able to restore them if something went wrong.
2. Open Google Apps Script and create a new project Calendar Synchronization.
3. Click at the + next to Services, add Google Calendar API v3 as Calendar.
4. Replace the Code.gs file content with this code and save.

Usage

The following examples are based on assumed calendars Work and Family.

Synchronization

1. Click the + next to Files to add a new script file onStart:

   function onStart() {
     sync('Work', 'Family')
   }

2. Select file Code.gs, run the function start() and grant the requested rights.

   Now, any change to the Work calendar is synchronized to the Family calendar.

3. To stop the synchronization, select file Code.gs and run stop().

4. To remove all synchronized events, select file Code.gs and run clean().

Multiple Source Calendars

Multiple source calendars can be synchronized to the same target calendar.

function onStart() {
  sync('Work', 'Family')
  sync('Second Work', 'Family')
}

Multiple Target Calendars

One source calendar can be synchronized to multiple target calendars.

function onStart() {
  sync('Work', 'Family')
  sync('Work', 'Second Family')
}

Time Range

The synchronization time range can be specified.

By default, the past 7 and next 28 days are synchronized.

sync('Work', 'Family', 7, 28)

There are a couple of helper function available to support the correction function.

For past days:

startOfWeek(offset = 0)       
startOfMonth(offset = 0)
startOfQuarter(offset = 0)
startOfHalfyear(offset = 0)
startOfYear(offset = 0)

For next days:

endOfWeek(offset = 0)         
endOfMonth(offset = 0)
endOfQuarter(offset = 0)
endOfHalfyear(offset = 0)
endOfYear(offset = 0)

Example - timerange is from the beginning of this week until the end of next month:

sync('Work', 'Family', startOfWeek(), endOfMonth(1))

Correction Function

To avoid any unwanted data exposure, by default, only the start date, end date, recurrence rule and Busy as default summary are synchronized.

The correction function can be specified to modify or filter target events.

function correctionFunction(targetEvent, sourceEvent) {
  // correction of the target event ...
  return targetEvent
}

sync('Work', 'Family', 7, 28, correctionFunction)

Use Cases

function correctionFunction(targetEvent, sourceEvent) {

  // Keep properties
  targetEvent.summary = sourceEvent.summary
  targetEvent.location = sourceEvent.location

  // Modify properties
  if (sourceEvent.attendees) targetEvent.colorId = '6'

  // Exclude events
  if (sourceEvent.transparency === 'transparent') targetEvent.status = 'cancelled'

  // Do not forget to return the target event
  return targetEvent

}

Read more about event properties and colors.

Helper Functions

There are a couple of helper function available to support the correction function.

isSynchronizedEvent(sourceEvent) // true if synchronized from another calendar
isRecurringEvent(sourceEvent)    // true if recurring event series or instance
isOOOEvent(sourceEvent)          // true if out of office event
isAlldayEvent(sourceEvent)       // true if allday event
isOnWeekend(sourceEvent)         // true if on Saturday or Sunday
isBusyEvent(sourceEvent)         // true if status is busy
isOpenByMe(sourceEvent)          // true if needs action by me
isAcceptedByMe(sourceEvent)      // true if accepted by me
isTentativeByMe(sourceEvent)     // true if responded tentative by me
isDeclinedByMe(sourceEvent)      // true if declined by me

Exclude synchronized events

By default, all source events are synchronized - also the ones which are synchronized from another calendar. You might want to exclude these events from your synchronization.

Example - this will result in a growing number of events:

function onStart() {
  sync('Work', 'Family')
  sync('Family', 'Work')
}

Solution - exclude synchronized events:

function correctionFunction(targetEvent, sourceEvent) {
  if (isSynchronizedEvent(sourceEvent)) targetEvent.status = 'cancelled'
  return targetEvent
}

function onStart() {
  sync('Work', 'Family', 7, 28, correctionFunction)
  sync('Family', 'Work', 7, 28, correctionFunction)
}

Sync Interval

The synchronization interval can be specified.

By default, the next synchronization is triggered after 1 minute.

This value can be increased, if the Google Calender API quota is an issue.

function onStart() {
  setSyncInterval(1)
  sync('Work', 'Family')
} 

Maximum Execution Time

A fallback trigger is starting the synchronization again, if the synchronziation script is not completed within the maximum execution time, given by Google Scripts.

By default, the maximum execution time is 6 minutes.

This value can be increased to make use of the increased Google Workspace limit of 30 minutes.

function onStart() {
  setMaxExecutionTime(30)
  sync('Work', 'Family')
} 

Update

To update the script version, replace the Code.gs file content with this code.

Deinstallation

1. To stop the synchronization, select file Code.gs and run stop().
2. To remove all synchronized events, select file Code.gs and run clean().
3. Remove the Google Apps Script project.

Support

Feel free to open an issue for bugs, feature requests or any other question.