title: Extending Timeindex by adding new values in another column in Pandas.
body: <p>Assume that I have the following dataframe in Pandas:</p>
  
  <pre><code>index = pd.date_range(start=pd.Timestamp('2017-07-01'), end=pd.Timestamp('2017-07-01') + pd.Timedelta('10D'))
  df = pd.DataFrame(data=np.random.rand(11), index=index , columns=['rand'])
  df.head()
  
                rand
  2017-07-01 0.794164
  2017-07-02 0.948194
  2017-07-03 0.432187
  2017-07-04 0.750968
  2017-07-05 0.591830
  </code></pre>
  
  <p>I want to add ten new values to <code>rand</code> column by extending index column by the number of added values i.e. until <code>2017-07-19</code>. I wonder if there is any way that the time index can be extended automatically only by adding values to <code>rand</code> column.</p>