Every single one of the hundred of millions of iPhone users out there is actively encouraged to have an iCloud account by the Apple on-boarding process. Access to the iCloud calendar that is then enabled on their iPhones relies on both an understanding of CalDAV as well as how Apple handles authentication.
The only API into Apple Calendar is CalDAV. This is a client/server document protocol that is used to submit and retrieve iCalendar documents. CalDAV can be a challenge for developers used to more commonly styled HTTP APIs.
With the number of Apple devices growing rapidly year on year, making apps and other software integrate with iCloud is increasingly important. However, working with Apple’s Calendar API is a challenge due to: