title: Calculate time difference on only months and days
body: <p>I would like to calculate the time difference considering ONLY the days and months.
  For example:</p>
  
  <pre><code>&gt; as.Date("2018-12-15")-as.Date("2018-12-16")
  Time difference of -1 days
  
  &gt; as.Date("2008-12-15")-as.Date("2018-12-16")
  Time difference of -3653 days
  </code></pre>
  
  <p>I want both of them to return -1.</p>
  
  <p>Edit:
  Leap years should not be considered as we just want an approximation and the return value do not need to be exact.</p>