title: pandas: retain hour while setting a constant for date, minutes
body: <p>I want to create a new column of datetime by setting a constant for the year, month, day, and minutes, seconds, retaining only the </p>
  
  <pre><code>import pandas as pd
  td = pd.DataFrame(['2015-01-01 09:03:00', '2015-01-11 15:47:00', 
              '2015-01-11 16:47:00', '2015-01-11 01:47:00', '2016-01-11 01:47:00'], columns=['datetime'])
  td['datetime'] = pd.to_datetime(td['datetime'])
  
  
      datetime
  0   2015-01-01 09:03:00
  1   2015-01-11 15:47:00
  2   2015-01-11 16:47:00
  3   2015-01-11 01:47:00
  4   2016-01-11 01:47:00
  </code></pre>
  
  <p>The result should look like this.</p>
  
  <pre><code>    datetime
  0   1900-01-01 09:00:00
  1   1900-01-01 15:00:00
  2   1900-01-01 16:00:00
  3   1900-01-01 01:00:00
  4   1900-01-01 01:00:00
  </code></pre>
  
  <p>How can I code this out? Thanks!</p>