title: Sort a pandas dataframe based on DateTime field
body: <p>I am trying to sort a dataframe based on DateTime field which is of datatype datetime64[ns].</p>
  
  <p>My dataframe looks like this:</p>
  
  <pre><code>Name    DateTime1
  P38     NaT
  P62     2016-07-13 16:03:32.771
  P59     2016-06-23 14:23:42.461
  P07     NaT
  P16     2016-06-23 14:02:06.237
  P06     2016-07-13 16:03:52.570
  P106    2016-07-13 19:56:22.676
  </code></pre>
  
  <p>When I sort it using DateTime field, </p>
  
  <pre><code>df.sort_values(by='DateTime1',ascending=True)
  </code></pre>
  
  <p>I do not get the desired result. </p>
  
  <p>Output:</p>
  
  <pre><code>Name    DateTime1
  P16     2016-06-23 14:02:06.237
  P59     2016-06-23 14:23:42.461
  P62     2016-07-13 16:03:32.771
  P06     2016-07-13 16:03:52.570
  P106    2016-07-13 19:56:22.676
  P38     NaT
  P07     NaT
  </code></pre>