title: difference between column of dates and a fixed date
body: <pre><code>shoot &lt;- read.csv('shoot.csv')
  finish &lt;- as.Date(shoot$`Year of Incorporation`, format="%mm/%dd/%yyyy")
  start &lt;- 12/31/2016
  date_diff &lt;- as.data.frame(finish-start)
  </code></pre>
  
  <p>What changes should be done in this to get output as number of dates between data set and <code>31/12/2016</code>??</p>