title: R calculate month end
body: <p>I have a question about date end, I explain in the following.<br>
  Here is my example data:  </p>
  
  <pre><code>DATE
  2015-01-01
  2015-02-05
  2015-09-29
  2016-02-07
  2016-07-24
  2016-12-16
  </code></pre>
  
  <p>I know if I want the total days in that month, the code is:  </p>
  
  <pre><code>days_in_month(DATE)
  </code></pre>
  
  <p>However, what I want is as following:  </p>
  
  <pre><code>DATE            DATE_Month_End
  2015-01-01      2015-01-31
  2015-02-05      2015-02-28
  2015-09-29      2015-09-30
  2016-02-07      2016-02-29
  2016-07-24      2016-07-31
  2016-12-16      2016-12-31
  </code></pre>
  
  <p>any suggestion?</p>