title: Strange DateTime::diff() result
body: <p>Can somebody please explain me the result of DateTime::diff() in the following example:</p>
  
  <pre><code>$start = DateTime::createFromFormat('Y/m/d', '2013/05/11');
  $end = DateTime::createFromFormat('Y/m/d', '2015/03/08');
  $diff = $start-&gt;diff($end);
  var_dump($diff);
  exit;
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>object(DateInterval)#19 (15) {
    ["y"]=&gt;
    int(1)
    ["m"]=&gt;
    int(9)
    ["d"]=&gt;
    int(25)
    ["h"]=&gt;
    int(0)
    ["i"]=&gt;
    int(0)
    ["s"]=&gt;
    int(0)
    ["weekday"]=&gt;
    int(0)
    ["weekday_behavior"]=&gt;
    int(0)
    ["first_last_day_of"]=&gt;
    int(0)
    ["invert"]=&gt;
    int(0)
    ["days"]=&gt;
    int(666)
    ["special_type"]=&gt;
    int(0)
    ["special_amount"]=&gt;
    int(0)
    ["have_weekday_relative"]=&gt;
    int(0)
    ["have_special_relative"]=&gt;
    int(0)
  }
  </code></pre>
  
  <p>This is what I think is happened here: 
  $diff->y is 1 because there is one <em>full</em> year between dates;
  $diff->m is 9 because there are 9 <em>full</em> months between dates (without 2013/05 and 2015/03).</p>
  
  <p>I'd like to know how number 25 is calculated. I can'f figure out where this number comes from.</p>
  
  <p>I'd expect here the difference between 2013/05/11 and 2013/05/31 + 2015/03/01 and 2015/03/08, so 20 + 7 = 27. Instead I got 25.</p>