title: Select rows from pandas DataFrame of a certain date
body: <p>I am fairly new to pandas and getting a problem with manipulating a DataFrame.</p>
  
  <p>What I have is a DataFrame with repeating dates and I would like to only keep the dates which are hourly.</p>
  
  <p>Here is an example of my current DataFrame:</p>
  
  <pre><code>Time                     ColA ColB
  5/12/2011 10:00:00 PM     9     4 
  5/12/2011 10:15:00 PM     5     3 
  5/12/2011 10:30:00 PM     1     1 
  5/12/2011 10:45:00 PM     2     3 
  5/12/2011 11:00:00 PM     10    4 
  </code></pre>
  
  <p>Thus the result should be a DataFrame that contains the first and last row alone.</p>
  
  <pre><code>Time                     ColA ColB
  5/12/2011 10:00:00 PM     9     4
  5/12/2011 11:00:00 PM     10    4
  </code></pre>