Cronofy

With the Cronofy Calendar API, developers work with just one API while building software that works with Google Calendar, Apple Calendar, Office 365, Outlook.com, and Microsoft Exchange.

Calendar events can even be synced across different calendar services when using Cronofy, making it easier than ever for your customers to book appointments with their clients.

The Cronofy Calendar API works with old and new versions of each calendar service. It's fully documented and easy-to-use.