title: get previous value of pandas datetime index
body: <p>I have a pandas dataframe with datetime index </p>
  
  <pre><code>Date
  2013-02-22 00:00:00+00:00    0.280001
  2013-02-25 00:00:00+00:00    0.109999
  2013-02-26 00:00:00+00:00   -0.150000
  2013-02-27 00:00:00+00:00    0.130001
  2013-02-28 00:00:00+00:00    0.139999
  Name: MOM12
  </code></pre>
  
  <p>and want to evaluate the previous three values of the given datetime index.</p>
  
  <pre><code>date = "2013-02-27 00:00:00+00:00"
  df.ix[date]
  </code></pre>
  
  <p>I searched for this but since my index is a date I can't do</p>
  
  <pre><code>df.ix[int-1]
  </code></pre>