title: Convert date in form of "%Y%M%D" to "%Y%M%D%H%M%S" in pandas
body: <p>Below is my dataframe</p>
  
  <pre><code>   Date Time   Open   High    Low  Close     Volume  Adj Close
  0 2012-05-15  15.90  16.06  15.80  16.02  134454.78       0.08
  1 2012-05-16  16.00  16.00  15.75  15.76  150305.11      -0.26
  2 2012-05-17  15.80  15.97  15.76  15.94  133389.98       0.18
  3 2012-05-18  15.80  15.91  15.66  15.72  157863.19      -0.22
  4 2012-05-21  15.72  15.84  15.70  15.77   85694.85       0.05
  </code></pre>
  
  <p>You see in the column of "Date Time", the data is in this form"%Y%M%D"
  I want to convert them to be "%Y%M%D%H%M$S" which is for example <code>2012-05-17 00:00:00</code> </p>
  
  <p>Thanks!</p>