title: datetimeindex.floor remove HH:MM:SS information
body: <p>When I perform <code>df.index.floor('12H')</code> on following two datasets, I get different output format.  </p>
  
  <pre><code>                     dummy
  2015-01-04 00:00:00      1
  2015-01-04 02:00:00      1
              dummy
  2015-01-04      1
  2015-01-04      1
                       dummy
  2015-01-04 00:00:00      1
  2015-01-04 12:00:00      1
  2015-01-04 02:00:00      1
  2015-01-04 13:00:00      1
  2015-01-04 22:00:00      1
                       dummy
  2015-01-04 00:00:00      1
  2015-01-04 12:00:00      1
  2015-01-04 00:00:00      1
  2015-01-04 12:00:00      1
  2015-01-04 12:00:00      1
  </code></pre>
  
  <p>How do I keep long time format "YYYY-MM-DD HH:MM:SS" for the first dateset, so that I can further merge this table with others.</p>
  
  <p>This is what I would like to have:</p>
  
  <pre><code>                     dummy
  2015-01-04 00:00:00      1
  2015-01-04 02:00:00      1
                       dummy
  2015-01-04 00:00:00      1
  2015-01-04 00:00:00      1
  </code></pre>