title: Extract dates in rows of panda dataframe when column value below certain range
body: <p>I have a panda dataframe <code>df</code> with the contents below;</p>
  
  <pre><code>       Date       Factor    Expiry         Grade  
  0 12/31/1991    2.138766   3/30/1992      -3.33% 
  1 10/29/1992    2.031381    2/8/1993      -1.06% 
  2  5/20/1993    2.075670    6/4/1993      -6.38% 
  3 10/11/1994    1.441644  11/22/1994      -7.80% 
  4  1/11/1995    1.669600   1/20/1995      -7.39% 
  5  5/15/1995    1.655237    8/8/1995      -8.68% 
  6 10/17/1996    0.942000  10/22/1996      -7.39% 
  7  2/19/1998    0.838838   5/26/1998      13.19% 
  8   7/9/1998    1.303637   8/28/1998      -6.73% 
  9 12/29/1998    1.517232   1/21/1999     -11.03% 
  10 4/26/1999    1.613346   5/24/1999      -7.55% 
  11  7/8/1999    2.136339   9/23/1999       5.43% 
  12 3/22/2000    5.097782   3/29/2000      -6.44% 
  </code></pre>
  
  <p>I would like to extract out dates under the <code>Date</code> column corresponding to the row with <code>Grade</code>  &lt;=-8%.</p>
  
  <p>The desirable output will be a list of string like this;</p>
  
  <pre><code>output_dates = ['5/15/1995', '12/29/1998']
  </code></pre>
  
  <p>I am using python v3.6</p>