A super simplified sample on a calendar view model which changes its values based on user's selection.

SimpleCalendarViewModel.swift

import Foundation

class CalendarViewModel {
	
	var selectedCheckInDate: NSDate?
	var selectedCheckOutDate: NSDate?
	
	func update(selectedDate date: NSDate) {
		
		switch (selectedCheckInDate, selectedCheckOutDate) {
		case (.none, .none):
			selectedCheckInDate = date
		case (.some(_), .none):
			selectedCheckOutDate = date
		case (.some(_), .some(_)):
			selectedCheckOutDate = nil
			selectedCheckInDate = date
		default:
			break
		}
	}
}