title: Difficulty using Diff on pandas date time series
body: <p>I have a series in Pandas (dtype = datetime64) as follows:</p>
  
  <pre><code>timeSeries
  35064   2013-11-11 16:44:26
  35065   2013-11-11 17:37:15
  35066   2013-11-11 18:30:03
  35067   2013-11-11 19:22:51
  35068   2013-11-11 20:15:39
  </code></pre>
  
  <p>If I then take the diff of this series:</p>
  
  <pre><code>diff_o_Series = diff(timeSeries)
  </code></pre>
  
  <p>I get the following results (dtype = timedelta64
  ):</p>
  
  <pre><code>diff_o_Series 
  35064        NaT
  35065   00:00:00
  35066   00:00:00
  35067   00:00:00
  35068        NaT
  </code></pre>
  
  <p>Which is clearly NOT the delta differences between the times in the original array.  Any thoughts on what I am doing wrong?</p>