title: Number of months between two dates
body: <p>Is there a standard/common method/formula to calculate the number of months between two dates in R?</p>
  
  <p>I am looking for something that is similar to <a href="http://www.mathworks.com/access/helpdesk/help/toolbox/finance/months.html" rel="noreferrer">MathWorks months function</a></p>