title: How to reindex a a pandas dataframe by date range without deleting values
body: <p><strong>Background:</strong></p>
  
  <p>I have the following data frame downloaded with pyodbc with dates between 1999 and 2015: </p>
  
  <pre><code>CEISales.head(10)
  Out[194]: 
     Order_DateC   RegionC     SalesC
  0  2014-01-30  Domestic    3530.00
  1  2011-10-11  Domestic     136.00
  2  1999-01-13  Domestic      30.00
  3  1999-01-13  Domestic   55615.00
  4  1999-01-13  Domestic     440.00
  5  1999-01-13  Domestic      94.00
  6  1999-01-05  Domestic     612.00
  7  1999-01-14  Domestic    1067.00
  8  1999-01-14  Domestic   26345.05
  9  1999-01-15  Domestic  161858.72
  </code></pre>
  
  <p>I then filtered the data for all dates greater than 2010-01-01 and sorted by ascending dates:</p>
  
  <pre><code>CEIFilter = CEISales[CEISales['Order_DateC'] &gt; '2010-01-01']
  
  CEITest = CEIFilter.sort('Order_DateC')
  
  CEITest.head(5)
  Out[199]: 
        Order_DateC   RegionC   SalesC
  18156  2010-01-04   Foreign    450.0
  18155  2010-01-04  Domestic   1990.4
  18154  2010-01-04  Domestic  37477.0
  18152  2010-01-04  Domestic      0.0
  18153  2010-01-04  Domestic    783.0
  </code></pre>
  
  <p>I then created a date index with values between 2010-01-01 and today with pandas's date_range function:</p>
  
  <pre><code>date_index = pd.date_range(start='2010-01-01', end='2015-12-23' , freq='d')
  </code></pre>
  
  <p>And re-indexed the data frame</p>
  
  <pre><code>CEIFinal= CEITest.reindex(date_index)
  </code></pre>
  
  <p>My problem is that when I re-indexed the data frame, all of the data was deleted:</p>
  
  <pre><code>CEIFinal.head(5)
  Out[206]: 
              Order_DateC RegionC  SalesC
  2010-01-01         NaT     NaN     NaN
  2010-01-02         NaT     NaN     NaN
  2010-01-03         NaT     NaN     NaN
  2010-01-04         NaT     NaN     NaN
  2010-01-05         NaT     NaN     NaN
  </code></pre>
  
  <p>From the original filtered data frame you can see that there were transactions on the 2010-04-01</p>
  
  <pre><code>CEITest[CEITest['Order_DateC'] == '2010-01-04']
  Out[210]: 
        Order_DateC   RegionC   SalesC
  18156  2010-01-04   Foreign    450.0
  18155  2010-01-04  Domestic   1990.4
  18154  2010-01-04  Domestic  37477.0
  18152  2010-01-04  Domestic      0.0
  18153  2010-01-04  Domestic    783.0
  </code></pre>
  
  <p><strong>Question</strong>
  How do I re-index this data frame with this date range and keep all of the original values? I am trying to create a common index on several different data frames from difference databases to add together into an aggregated data frame. Your help would be greatly appreciated. Thanks!</p>