title: Count number of observations per day, month and year in R
body: <p>I have the a dataframe in the following form (its too big to post here entirerly):</p>
  
  <pre><code>      listing_id    date    city    type    host_id availability
  1   703451  25/03/2013  amsterdam   Entire home/apt 3542621 245
  2   703451  20/04/2013  amsterdam   Entire home/apt 3542621 245
  3   703451  28/05/2013  amsterdam   Entire home/apt 3542621 245
  4   703451  15/07/2013  amsterdam   Entire home/apt 3542621 245
  5   703451  30/07/2013  amsterdam   Entire home/apt 3542621 245
  6   703451  19/08/2013  amsterdam   Entire home/apt 3542621 245
  </code></pre>
  
  <p>and so on...</p>
  
  <p>I would like three new data frames. One counting the number of observations for a particular year (2013,2012, 2011 and so on) another per month (07/2013, 06/2013 and so on) and another per day (28/05/2013, 29/05/2013 and so on). 
  I just want to count how many occurances there are per unit of time.</p>
  
  <p>How would I do that?</p>