title: How to determine when multiple(n) datetime ranges overlap each other
body: <p>I'm looking for a function that I can call that will tell me what datetime ranges all (p) people are available at the same time.
  Please help me for Objective-c or Swift language.</p>
  
  <pre><code>p1: start: "2016-01-01 12:00", end: "2016-05-01 03:00"
  p2: start: "2016-01-01 03:00", end: "2016-05-01 03:00"
  p3: start: "2016-01-01 03:00", end: "2016-04-30 13:31"
  </code></pre>
  
  <p>In the above example the answer should be:</p>
  
  <pre><code>start: 2016-04-30 12:00, end: 2016-04-30 13:31
  </code></pre>