From The Apple Wiki
Original author(s)Apple Inc.
Developer(s)Apple Inc.
Stable release
(latest known version)
Operating systemiOS
Available inEnglish
LicenseClosed source

Guest Book is an internal application presumably used at Apple Stores to note down Genius Bar appointments and walk-ins. Its bundle has a file named appointment.json which most likely contains a formatting example for how a booked appointment would be stored. The contents of the file are:

   "appointmentId": 0,
   "appleId": 567898,
   "scheduleId": 569,
   "startTime": "2018-12-11T08:00:00Z",
   "endTime": "2018-12-11T20:00:00Z",
   "customerName": "John Doe",
   "customerEmail": "",
   "customerPhone": "6073793577",
   "status": 0,
   "cancelationReason": 0,
   "notes": "Some notes on the customer",
   "isBlocked": false