title: Using strtotime() for mathematical operations is not advisable?
body: <p>While reading the PHP manual for the strtotime() function I got to a "Note" with a very poor explanation - saying this: </p>
  
  <blockquote>
    <p><strong>Note:</strong><br>
    Using this function for mathematical operations is not advisable. It is better to use DateTime::add() and DateTime::sub() in PHP 5.3 and later, or DateTime::modify() in PHP 5.2.</p>
  </blockquote>
  
  <p>What is meant by the "mathematical operations"? <br>...of course I know what mathematical operations are in general, but in what context is this meant?</p>
  
  <p>And why it is not "advisable"?</p>
  
  <p>Could anyone show me an example of use where it "is not advisable" and explain why?</p>
  
  <p>Thanks.</p>