Reschedule calendar appointments with Google Sheets and Google Calendar

Description

Quick overview
This workflow is triggered by another n8n workflow to reschedule existing appointment by looking up current booking in Google Sheets, validating the requested new slot via sub-workflow, and updating the event in Google Calendar with the new date/time & service details. Full setup guide: https://uzzaman.dev/guides/n8n-google-calendar-reschedule-appointments/

How it works
Receives appointment details (phone, current date/time, new date/time, and new service type) from another workflow.
Looks up the existing booked appointment in Google Sheets using the phone number and the current (preferred) date and time.
If a matching appointment is found, builds the new start and end timestamps for the requested slot.
Calls a separate n8n workflow to check whether the requested new slot is available.
If the slot is available, updates the corresponding Google Calendar event with the new start/end time, updated summary, and phone number in the description.
Returns a structured result indicating success (Rescheduled=true plus formatted date/time) or failure with an error such as appointment_not_found, new_slot_unavailable, calendar_unavailable, or crm_unavailable.

Setup
Connect Google Sheets OAuth credentials and replace YOUR_GOOGLE_SHEET_ID with your spreadsheet ID, ensuring the sheet contains columns for preferred_date, preferred_time, status, phone, event_id, and name.
Connect Google Calendar OAuth credentials and select the target calendar email address used to store the appointments.
Create and link the “Check Slot Availability” sub-workflow so it accepts preferred_date and preferred_time and returns an available boolean.
Ensure the calling workflow passes inputs in the expected formats (phone in E.164, new_date as YYYY-MM-DD, and new_time as HH:mm), and adjust the default 30-minute duration if your appointments differ.