title: Filter Pandas DataFrame by time index
body: <p>I have a pandas DataFrame from 6:36 AM to 5:31 PM.  I want to remove all observations where the time is less than 8:00:00 AM.  Here is my attempt:</p>
  
  <pre><code>df = df[df.index &lt; '2013-10-16 08:00:00']
  </code></pre>
  
  <p>This does nothing, please help.</p>