title: pandas dataframe sort by date
body: <p>I made a dataframe by importing a csv file. And converted the date column to datetime and made it the index. However, when sorting the index it doesn't produce the result I wanted</p>
  
  <pre><code>print(df.head())
  df['Date'] = pd.to_datetime(df['Date'])
  df.index = df['Date']
  del df['Date']
  df.sort_index()
  print(df.head())
  </code></pre>
  
  <p>Here's the result:</p>
  
  <pre><code>         Date     Last
  0  2016-12-30  1.05550
  1  2016-12-29  1.05275
  2  2016-12-28  1.04610
  3  2016-12-27  1.05015
  4  2016-12-23  1.05005
                 Last
  Date               
  2016-12-30  1.05550
  2016-12-29  1.05275
  2016-12-28  1.04610
  2016-12-27  1.05015
  2016-12-23  1.05005
  </code></pre>
  
  <p>The date actually goes back to 1999, so if I sort this by date, it should show the data in ascending order right?</p>