title: Starting iterrows() loop on a particular index
body: <p>I'm using <code>iterrows()</code> to do calculations on a dataset that uses a timestamp as its index.  See below as an example:</p>
  
  <pre><code>&gt;&gt;&gt; aapl.head()
  Date        Open   High    Low  Close    Volume  Adj Close                                                        
  1980-12-12  28.75  28.88  28.75  28.75  16751200       3.15
  1980-12-15  27.38  27.38  27.25  27.25   6281600       2.99
  1980-12-16  25.38  25.38  25.25  25.25   3776000       2.77
  1980-12-17  25.88  26.00  25.88  25.88   3087200       2.84
  1980-12-18  26.62  26.75  26.62  26.62   2623200       2.92
  </code></pre>
  
  <p>I was planning on doing my calculation using <code>for index in aapl.iterrows()</code> but I'm only looking at values within a specific year and the dataset contains much more data than that.  How might I get my loop to start on a particular index value (namely timestamp in this case) and end on another?</p>
  
  <p>Many thanks for any suggestions.</p>