EventKit is a framework on iOS and OS X which provides classes for accessing and modifying calendar event information.
From the documentation:
The Event Kit and Event Kit UI frameworks together allow applications to access event information from a user’s Calendar database. You can fetch events based on a date range or a unique identifier, receive notifications when event records change, and allow users to create and edit events for any of their calendars. Changes made to events in a user’s Calendar database with Event Kit are automatically synced with the appropriate calendar (CalDAV, Exchange, and so on)
The EventKit framework provides classes for accessing and manipulating calendar events and reminders.
Event Kit not only allows your app to retrieve users’ existing calendar and reminder data, but it also lets your app create new events and reminders for any of their calendars. In addition, Event Kit lets users edit and delete their events and reminders (collectively known as “calendar items”). More advanced tasks, such as adding alarms or specifying recurring events, can be achieved with Event Kit as well. If a change to the Calendar database occurs from outside of your app, Event Kit is able to detect the change by notification so your app can act appropriately. Changes made to calendar items with Event Kit are automatically synced to the associated calendar (CalDAV, Exchange, and so on).
Event Kit provides limited access to a user’s Calendar database; it does not include everything that would be desired for implementing a full-featured calendar or reminder app, such as adding attendees or accounts.
Reference:
1.EventKit Constants Reference
2.EventKit Framework Reference
3.Introduction to Calendars and Reminders
4.EventKit UI Framework Reference
Sample Code: