Google Apps Script for Consolidating Many Calendars into One

Code.gs

const now = new Date();
const thirtyDaysFromNow = new Date(now.getTime() + (30 * 24 * 60 * 60 * 1000));

const sourceCalendarIds = [] // array of ids of calendars you want to consolidate
const targetCalendarId = '' // string of your target calendar id (i.e. the calendar you want to consolidate events to)

const sourceCalendars = sourceCalendarIds.map(id => CalendarApp.getCalendarById(id))
const targetCalendar = CalendarApp.getCalendarById('') 

function sync() {
  Logger.log('Starting sync...')

  // Get existing events from the target calendar
  const existingEvents = targetCalendar.getEvents(now, thirtyDaysFromNow)
  const existingEventsKeys = new Set(existingEvents.map(e => `${e.getStartTime()}#####${e.getEndTime()}`))
  Logger.log('existing events')
  Logger.log(existingEventsKeys.size)

  // Get the events from all of the source calendars
  const sourceEvents = sourceCalendars.reduce((acc, cal) => {
    const events = cal.getEvents(now, thirtyDaysFromNow)
    return [...acc, ...events]
  }, [])
  const sourceEventsKeys = new Set(sourceEvents.map(e => `${e.getStartTime()}#####${e.getEndTime()}`))
  Logger.log('source events')
  Logger.log(sourceEventsKeys.size)

  // Determine the events that should be removed from target calendar
  const eventsToDeleteKeys = new Set([...existingEventsKeys].filter(x => !sourceEventsKeys.has(x)))
  const eventsToDelete = existingEvents.filter(e => eventsToDeleteKeys.has(`${e.getStartTime()}#####${e.getEndTime()}`))
  Logger.log('events to delete')
  Logger.log(eventsToDeleteKeys.size)

  // Determine the events that should be added to the target calendar
  const eventsToCreateKeys = new Set([...sourceEventsKeys].filter(x => !existingEventsKeys.has(x)))
  Logger.log('events to create')
  Logger.log(eventsToCreateKeys.size)

  // Delete the events that should be removed from the target calendar
  eventsToDelete.forEach((e, i) => {
    if (i % 10 === 0) Utilities.sleep(3000) // to deal with throttling of requests by Google
    Logger.log(`Deleting outdated event: ${e.getStartTime()}, ${e.getEndTime()}`)
    e.deleteEvent()
  })

  // Create the events that should be added to the target calendar
  Array.from(eventsToCreateKeys).forEach((e, i) => {
    Logger.log(i)
    if (i % 10 === 0) Utilities.sleep(3000) // to deal with throttling of requests by Google
    const [start, end] = e.split('#####')
    targetCalendar.createEvent('Not available', new Date(start), new Date(end))
    Logger.log(`New event created for ${new Date(start)}, ${new Date(end)}`)
  })
}