title: Filter rows by condition applied to indices
body: <p>I have a DataFrame with dates as indices:</p>
  
  <pre><code>            VL
  2018-02-05  101.56093
  2018-12-31  95.87728
  2019-01-04  96.29820
  2019-01-11  97.23475
  2019-01-18  98.39828
  2019-01-25  98.66896
  2019-01-31  99.12407
  2019-02-01  99.13224
  2019-02-08  99.06382
  2019-02-15  99.79966    
  </code></pre>
  
  <p>I need to filter the rows so that, <strong>for each row with date <code>D</code>, keep it if the row with <code>D-7</code> exists in the DataFrame</strong>.</p>
  
  <p>Example:</p>
  
  <ul>
  <li><p><code>2019-02-15</code> would remain, because <code>2019-02-08</code> is present</p></li>
  <li><p><code>2019-01-31</code> would be filtered as <code>2019-01-24</code> is not present.</p></li>
  </ul>
  
  <p>I've implemented this already using a loop but I'm wondering if there is a more pandas oriented way of doing this kind of filtering.</p>