title: Merge Pandas Dataframes based on date
body: <p>I'm looking to merge two pandas DataFrames based on date. The issue is the 2nd dataframe does not include every date from the 1st dataframe. I need to use every date from <code>df1</code> with the latest value from <code>df2</code>.</p>
  
  <pre><code>+-------------+---------------+-------------+
  | DataFrame 1 |               |             |
  +-------------+---------------+-------------+
  | Date        |  Sales loc1   |  Sales loc2 |
  | 1/1/17      |  100          |  95         |
  | 1/2/17      |  125          |  124        |
  | 1/3/17      |  115          |  152        |
  | ...         |               |             |
  | 2/1/17      |  110          |  111        |
  +-------------+---------------+-------------+
  
  
  +-------------+---------+------+
  | DataFrame 2 |         |      |
  +-------------+---------+------+
  | Date        |  exp    |  loc |
  | 1/1/17      |  100    |  1   |
  | 1/1/17      |  125    |  2   |
  | 2/1/17      |  115    |  1   |
  | 2/1/17      |  110    |  2   |
  +-------------+---------+------+
  
  
  +---------------+---------------+--------------+------------+-------------+
  | New Dataframe |               |              |            |             |
  +---------------+---------------+--------------+------------+-------------+
  | Date          |  Sales loc1   |  Sales loc2  |  exp loc1  |  exp loc2   |
  | 1/1/17        |  100          |  95          |  100       |  125        |
  | 1/2/17        |  125          |  124         |  100       |  125        |
  | 1/3/17        |  115          |  152         |  100       |  125        |
  | ...           |               |              |            |             |
  | 2/1/17        |  110          |  111         |  115       |  110        |
  +---------------+---------------+--------------+------------+-------------+
  </code></pre>
  
  <p>The values from df2 will be used for multiple cells till there is a new value in df2.</p>
  
  <p>Thanks a lot for your time. </p>