title: calculate difference between dates and add column for the result in data frame R
body: <p>I searched for a solution to my specific problem but I didn't find a conclusion.</p>
  
  <p>I have a dataframe which looks like </p>
  
  <pre><code>ID1  |   ID2 |     Date   |
   1   |   a   |  01/02/2018|
   1   |   b   |  03/05/2018|
   1   |   c   |  20/08/2018|
   2   |   ee  |  02/11/2017|
   2   |   ff  |  18/11/2017|
  </code></pre>
  
  <p>I want to calculate the difference between dates for each ID. 
  For example for <code>ID=1</code> I would like to have a column with the number of days: 91 (difference between 03/05/2018 and 01/02/2018) and 109 (difference between 20/08/2018 and 03/05/2018).</p>
  
  <p>My dataframe contains 4500 rows and the ids can be duplicated more than 2 times.</p>
  
  <p>I have already worked with <code>diff_date</code> to calculate the difference between dates in different columns.</p>
  
  <p>Thank you!</p>