title: How to obtain difference of a date column in groupby
body: <p>Currently my data looks like :</p>
  
  <pre><code>user_ID order_number    order_start_date    order_value week_day
  237 135950  1594878.0   2018-01-01  534.0   Monday
  235 32911   1594942.0   2018-01-01  89.0    Monday
  232 208474  1594891.0   2018-01-01  85.0    Monday
  231 9048    1594700.0   2018-01-01  224.0   Monday
  228 134896  1594633.0   2018-01-01  449.0   Monday
  </code></pre>
  
  <p>What I want to achieve is groupby the records by user_ID and take difference of min and max value of each date and find out difference between them in days. Where I am struggling:</p>
  
  <ul>
  <li>Groupby does not inherently supports minimum maximum difference</li>
  <li>It is not possible to perform numerical operations such as mean() on datetime series which exist as a column in a dataframe. Though possible for individual series. 
  Any help?</li>
  </ul>