Google App Script Shift Events Back by One Day

Google App Script Shift Events Back by One Day

/**
 * Shifts every event in a given Google Calendar back by exactly one day (24h).
 * - Automatically detects the true min/max date range of existing events.
 * - Includes a DRY_RUN mode so you can preview changes before applying.
 */
function shiftCalendarEventsBackOneDay_AutoRange() {

  /** 
   * 1. REQUIRED: Replace this with the Calendar ID you want to modify.
   *    You can find it under Google Calendar → Settings → "Calendar ID"
   *    Example: "abc123@group.calendar.google.com"
   */
  const CALENDAR_ID = "YOUR_CALENDAR_ID_HERE";

  /**
   * 2. Safety toggle:
   *    - true  = only LOG what would change (does NOT edit events)
   *    - false = actually modify the events
   *    Always run at least once in DRY_RUN mode before setting to false.
   */
  const DRY_RUN = true;

  // 3. Get the Calendar object by ID
  const cal = CalendarApp.getCalendarById(CALENDAR_ID);
  if (!cal) throw new Error("Calendar not found. Check CALENDAR_ID.");

  /**
   * 4. Fetch all events using a very wide date window.
   *    We don't know the real range yet, so we grab everything between 2000–2100.
   *    (This is safe—Google will only return real events that exist.)
   */
  const FAR_PAST   = new Date(2000, 0, 1);   // Jan 1, 2000
  const FAR_FUTURE = new Date(2100, 0, 1);   // Jan 1, 2100
  const events = cal.getEvents(FAR_PAST, FAR_FUTURE);

  // If no events exist, there's nothing to do
  if (events.length === 0) {
    Logger.log("No events found in the calendar.");
    return;
  }

  /**
   * 5. Find MIN start time and MAX end time in the calendar
   *    This gives us the real date range that imported events cover.
   */
  let minStart = events[0].getStartTime();
  let maxEnd   = events[0].getEndTime();

  events.forEach(event => {
    const start = event.getStartTime();
    const end   = event.getEndTime();

    if (start < minStart) minStart = start;
    if (end   > maxEnd)   maxEnd   = end;
  });

  // Log the detected date range so the user can confirm it looks correct
  Logger.log(`Detected calendar range: ${minStart} → ${maxEnd}`);
  Logger.log(`Total events detected: ${events.length}`);

  /**
   * 6. Define 1 day in milliseconds
   *    (Google Calendar stores times internally as full datetime objects,
   *    so we adjust via millisecond math!)
   */
  const ONE_DAY_MS = 24 * 60 * 60 * 1000;

  // 7. Track how many events were processed (for reporting)
  let count = 0;

  /**
   * 8. Loop through every event and push it back by one day
   */
  events.forEach(event => {

    // Original start/end times from calendar
    const oldStart = event.getStartTime();
    const oldEnd   = event.getEndTime();

    // New shifted times
    const newStart = new Date(oldStart.getTime() - ONE_DAY_MS);
    const newEnd   = new Date(oldEnd.getTime() - ONE_DAY_MS);

    if (DRY_RUN) {
      // Only log what WOULD happen — no changes made
      Logger.log(`[DRY RUN] "${event.getTitle()}"  ${oldStart} → ${newStart}`);
    } else {
      // Actually update the event in Google Calendar
      event.setTime(newStart, newEnd);
    }

    count++;
  });

  // 9. Final summary log
  Logger.log(`${DRY_RUN ? "Would shift" : "Shifted"} ${count} event(s) back by one day.`);
}