title: Create new Pandas column from existing column using previous date
body: <p>I have a dataset where I have a value for each month that looks like:</p>
  
  <pre><code>Date         Platform    Name   Value
  1-31-2019    Pine        Dave   100
  1-31-2019    Pine        Sam    300
  1-31-2019    Cherry      Sam    200
  2-28-2019    Pine        Dave   150
  2-28-2019    Pine        Sam    200
  2-28-2019    Cherry      Sam    250
  
  </code></pre>
  
  <p>I need to use the previous month's value to run calculations, and I'd like to add a column that is last month's value. So I'd like to have an end result that looks like</p>
  
  <pre><code>Date         Platform    Name   Value    Previous Value
  1-31-2019    Pine        Dave   100      NaN
  1-31-2019    Pine        Sam    300      NaN
  1-31-2019    Cherry      Sam    200      NaN
  2-28-2019    Pine        Dave   150      100
  2-28-2019    Pine        Sam    200      300
  2-28-2019    Cherry      Sam    250      200
  
  </code></pre>
  
  <p>I have the date columns set to a datetime data type. Is it possible to do a date calculation to create a new columns?</p>