title: Calendar add() vs roll() when do we use it?
body: <p>I know <code>add()</code> adds the specified (signed) amount of time to the given time field, based on the calendar's rules.</p>
  
  <p>And <code>roll()</code> adds the specified (signed) single unit of time on the given time field <strong>without changing larger fields.</strong></p>
  
  <p>I can't think of an everyday usage of <code>roll()</code> I would do everything by <code>add()</code>. </p>
  
  <p>Can you help me out with examples when do we use <code>roll()</code> and when <code>add()</code>?</p>
  
  <p><strong>EDIT 1</strong></p>
  
  <p><strong>Joda</strong> answers are not accepted!</p>