title: How to extract frequency per datetime stamp
body: <p>From a data frame as month and a continuous number id</p>
  <pre><code>data.frame( id = c(1,1,2,2,3), month = c(&quot;2011-02&quot;,&quot;2011-02&quot;,&quot;2011-02&quot;,&quot;2011-03&quot;,&quot;2011-01&quot;))
  </code></pre>
  <p>How is it possible to extract the total number of month and have the frq for every id?
  Expected output:</p>
  <pre><code>data.frame(id = c(1,2,2,3), month = c(&quot;2011-02&quot;,&quot;2011-02&quot;,&quot;2011-03&quot;,&quot;2011-01&quot;), frq = c(1,1,2,1)
  </code></pre>