title: How to calculate time difference between two pandas column
body: <p>My df looks like,</p>
  
  <pre><code>    start               stop
  0   2015-11-04 10:12:00 2015-11-06 06:38:00
  1   2015-11-04 10:23:00 2015-11-05 08:30:00
  2   2015-11-04 14:01:00 2015-11-17 10:34:00
  4   2015-11-19 01:43:00 2015-12-21 09:04:00
  
  print(time_df.dtypes)
  
  start       datetime64[ns]
  stop        datetime64[ns]
  </code></pre>
  
  <p>dtype: object</p>
  
  <p>I am trying to find the time difference between, stop and start.</p>
  
  <p>I tried, <code>pd.Timedelta(df_time['stop']-df_time['start'])</code>
  but it gives <code>TypeError: data type "datetime" not understood
  </code></p>
  
  <p><code>df_time['stop']-df_time['start']</code> also gives same error.</p>
  
  <p>My expected output,</p>
  
  <pre><code> 2D,?H
   1D,?H
   ...
   ...
  </code></pre>