title: How to do complex selection in pandas?
body: <p>I have a df like below:</p>
  
  <pre><code>President   Start Date  End Date
  B Clinton   1992-01-01  1999-12-31
  G Bush      2000-01-01  2007-12-31
  B Obama     2008-01-01  2015-12-31
  D Trump     2016-01-01  2019-12-31 # not too far away!!
  </code></pre>
  
  <p>I want to create another df, something like this</p>
  
  <pre><code>timestamp   President
  1992-01-01  B Clinton
  1992-01-02  B Clinton
  ...
  2000-01-01  G Bush
  ...
  </code></pre>
  
  <p>Basically I want to create a dataframe which its index is time stamp and then its content is selected based on the condition on the two columns of another df.</p>
  
  <p>I feel there is a way within pandas to do this, but I am not sure how. I tried to use <code>np.piecewise</code> but seems generating the conditions will be very hard for me. How could I do this?</p>