Open Account & plan, then Calendar feed, and copy your Subscription link. In Google Calendar on a computer, choose Other calendars, then From URL, and paste it. In Apple Calendar, choose File, then New Calendar Subscription. The calendar then shows your booked and completed bookings from today on, each titled with the customer and service and with the person and place in the notes, plus invoice due dates, quote expiry dates and the follow-ups FullDiary will send.
In the FullDiary app it is under More, in the settings: Add to Apple Calendar on an iPhone, Add to Google Calendar, and Share the link for any other calendar. Google only adds a calendar from a link on its website, not in its own app, so the app opens it in a browser.
It is a one-way view. Book and move appointments in FullDiary, not in the calendar. Calendar apps refresh a subscription on their own schedule, so a change can take a few hours to show up there.
Treat the link like a password: anyone who has it can see your bookings, customer names and amounts. If it gets out, Create a new link replaces it and the old one stops working.
Your customers have a calendar file of their own in their confirmation email.