Sync personal events to your work calendar

update-work-calendar.gs

// WHAT DOES THIS DO?
//
// If you follow the instructions below, your work calendar will show you as busy during events on
// your personal calendar.
//
//
// NOTES
//
// This code is based on [Jeremy Hon's code][1].
//
// The events this code creates on your work calendar will not be edited or removed
// if the original event on your personal calendar is edited or removed.
//
// You can synchronize any two calendars in this way, but these instructions assume
// you are copying events from your personal calendar to your work calendar. If needed,
// substitute PERSONAL and WORK to fit your situation.
//
//
// INSTRUCTIONS
//
// 1.  On your PERSONAL Google account, go to calendar.google.com.
// 2.  Click on the cog -> Settings.
// 3.  In the sidebar go to 'Settings for my calendars' -> click your PERSONAL calendar.
// 4.  Scroll to 'Share with specific people or groups'.
// 5.  Click "+ Add people and groups" and type in your WORK Google account email.
//     You can select any permissions you want; they all work.
//     No matter which you select, your synced events will only show "busy" to your coworkers.
// 6.  Scroll to 'Integrate calendar' and make note of your Calendar ID (if this is your
//     account's primary calendar then the ID is exactly your email address).
// 7.  Open a new tab. On your WORK Google account, go to calendar.google.com. In the sidebar,
//     click the '+' next to 'Other calendars', click 'Subscribe to calendar' and subscribe to
//     your PERSONAL Google account's calendar you just shared by typing its email address.
// 8.  On your WORK Google account, go to script.google.com.
// 9.  Click 'New project'. Delete the default contents and paste THIS FILE that you are
//     reading into the textarea. It's easier to copy-paste if you click "Raw" in the top-right.
// 10. Below these instructions you'll see some text that says "CHANGEME"; change this to your
//     calendar ID you noted from step 5.
// 11. Click 'Save project' (small button near the Undo/Redo buttons; do NOT click deploy)
//     then click 'Run'. Grant permission if asked.
// 12. You should see its progress in the log at the bottom.
//     When you see "Execution completed",
//     check your calendars to make sure it worked
//     (events in the next 30 days on your personal calendar
//     should appear on your work calendar as "busy").
//     YOU ARE NOT DONE YET!
// 13. In the left nav bar go to Triggers, then click Add Trigger.
//     1. Set 'Select event source' to 'From calendar'.
//     2. Write your PERSONAL email in the box.
//     3. Leave everything else default and click Save. Grant permission if asked.
// 14. Go to your PERSONAL Google Calendar and create an event to test; it should appear
//     within ~30 seconds on your work calendar as 'busy'!
//
// PS. If and when you leave your job, you can force undo this by repeating steps 1–4
//     then clicking the "X" by your work account.
//
// [1]: https://gist.github.com/jeremyhon/60b7be41273a04b98c339014804d3e28


function sync() {
  personalCalendarIds = [
    "CHANGEME",
  ]
    
  // How many days in advance to monitor and block off time for?
  const daysForward = 30
  
  // What should the copied events on your work calendar be called?
  const twinEventTitle = "busy"

  const workCal = CalendarApp.getDefaultCalendar()
  const personalCals = personalCalendarIds.map(id => CalendarApp.getCalendarById(id))

  if (personalCals.some(cal => cal === null)) {
    console.error("Couldn't fetch personal calendar. Did you remember to subscribe to it on your work account?")
    return
  }

  const today = new Date()
  const enddate = new Date()
  enddate.setDate(today.getDate() + daysForward)
  
  const workEvents = workCal.getEvents(today, enddate)
  const personalEvents = personalCals.map(cal => cal.getEvents(today, enddate)).flat()

  console.log(`Sifting through ${personalEvents.length} personal events over the next ${daysForward} days`)
  for (personalEvent of personalEvents) {
    if (workEvents.some(workEvent => (
        workEvent.getStartTime().getTime() == personalEvent.getStartTime().getTime() &&
        workEvent.getEndTime().getTime() == personalEvent.getEndTime().getTime()
      )
    )) {
      continue
    }
    
    const d = personalEvent.getStartTime()
    const n = d.getDay()

    // Skip all-day events. Delete this if you want to include all-day events.
    if (personalEvent.isAllDayEvent()) {
      continue
    }
    
    // Skip weekends. Delete this if you want to include weekends.
    if (n == 0 || n == 6) {
      continue
    }
    
    const newEvent = workCal.createEvent(twinEventTitle, personalEvent.getStartTime(), personalEvent.getEndTime())

    // alternative version below that copies the exact secondary event information into the primary calendar event
    // const newEvent = workCal.createEvent(personalEvent.getTitle(), personalEvent.getStartTime(), personalEvent.getEndTime(), {location: personalEvent.getLocation(), description: personalEvent.getDescription()})

    // Prevent the copies from sending "starts soon" notifications.
    newEvent.removeAllReminders()
  }
}

As the above seem to work for Google Calendars only, check out https://github.com/bjesus/callibella if you're looking for something non-Google :)