title: Finding number of NaT
body: <p>I'm trying to find a number of <code>Not A Time</code> values in a grouping:</p>
  
  <pre><code>ID    Date         eVal
  ddd   2014-02-12   2
  ddd   2014-02-13   2
  ddd   NaT          2
  aaa   2014-02-12   3
  aaa   2014-02-13   3
  aaa   2014-02-14   3
  </code></pre>
  
  <p>I basically need to add a new column which indicates the <code>(number of NaT incidences for that ID)</code></p>
  
  <p>How do I find that number?</p>