How to add an event in the device calendar using swift?

Note: If your app is crashing with This app has crashed because it attempted to access privacy-sensitive data without a usage description. The app's Info.plist must contain an NSCalendarsUsageDescription key with a string value explaining to the user how the app uses this data., you'll need to add NSCalendarsUsageDescription to your info.plist. Can follow the example here.

Swift 5.0 Version

import Foundation
import EventKit

let eventStore : EKEventStore = EKEventStore()
      
// 'EKEntityTypeReminder' or 'EKEntityTypeEvent'

eventStore.requestAccess(to: .event) { (granted, error) in
  
  if (granted) && (error == nil) {
      print("granted \(granted)")
      print("error \(error)")
      
      let event:EKEvent = EKEvent(eventStore: eventStore)
      
      event.title = "Test Title"
      event.startDate = Date()
      event.endDate = Date()
      event.notes = "This is a note"
      event.calendar = eventStore.defaultCalendarForNewEvents
      do {
          try eventStore.save(event, span: .thisEvent)
      } catch let error as NSError {
          print("failed to save event with error : \(error)")
      }
      print("Saved Event")
  }
  else{
  
      print("failed to save event with error : \(error) or access not granted")
  }
}   

Reference : https://gist.github.com/mchirico/d072c4e38bda61040f91

Swift 3.0 compatible:

func addEventToCalendar(title: String, description: String?, startDate: Date, endDate: Date, completion: ((_ success: Bool, _ error: NSError?) -> Void)? = nil) {
    let eventStore = EKEventStore()

    eventStore.requestAccess(to: .event, completion: { (granted, error) in
        if (granted) && (error == nil) {
            let event = EKEvent(eventStore: eventStore)
            event.title = title
            event.startDate = startDate
            event.endDate = endDate
            event.notes = description
            event.calendar = eventStore.defaultCalendarForNewEvents
            do {
                try eventStore.save(event, span: .thisEvent)
            } catch let e as NSError {
                completion?(false, e)
                return
            }
            completion?(true, nil)
        } else {
            completion?(false, error as NSError?)
        }
    })
}

And also import EventKit

So you can easily call this method from everywhere:

addEventToCalendar(title: "Girlfriend birthday", description: "Remember or die!", startDate: NSDate(), endDate: NSDate())

If you prefer, you can put this method inside an utiliy class and define it as 'static'.