title: How to get the interval value from two UIDatePicker
body: <p>I have two UIDatePickers. <code>startDatePicker</code> is to choose a start date and <code>endDatePicker</code> is to choose an end date. It should be something like</p>
  
  <p>START: 2015/06/25 11:00<br>
  END: 2015/07/06 13:00<br>
  ->Time Interval: 12 days 2 hours = 1044000 seconds</p>
  
  <p>And the 1044000 seconds is what I want to get here. Just I am not sure how.</p>
  
  <p>I am guessing maybe I can use<br>
  <code>startDatePicker.date.timeIntervalSinceDate(endDatePicker.date)</code><br>
  to get the value? If so, what is the necessary settings for the two UIDatePickers?</p>
  
  <p>Or maybe there are other ways to do that? Thanks in advance.</p>