title: PHP relative month not producing expected result
body: <p>So, I am working with subtracting relative months. According to the documentation:</p>
  
  <blockquote>
    <p>Relative month values are calculated based on the length of months that they pass through. An example would be "+2 month 2011-11-30", which would produce "2012-01-30". This is due to November being 30 days in length, and December being 31 days in length, producing a total of 61 days. </p>
  </blockquote>
  
  <p>I understand why the example when adding months, but I don't understand why <code>2014-12-31 -1 month</code> produces <code>2014-12-01</code>. Its 31 days in December, shouldn't the result be the last of November?</p>
  
  <p>Example code. Function 1 produces the result I was expecting from function 2 and 3.</p>
  
  <pre><code>//Func 1
  $date = '2014-12-31 23:59:59'; //YYYY-MM-DD
  $days = 0;
  echo $date.'&lt;br&gt;';
  for ($i = 1; $i &lt; 13; $i++) {
      $days += cal_days_in_month(CAL_GREGORIAN, (13-$i), 2014);
      echo date("Y-m-d H:i:s", strtotime("-$days days", strtotime($date))).'&lt;br&gt;';
  }
  echo "&lt;br&gt;&lt;br&gt;";
  //Func 2
  $date = '2014-12-31 23:59:59'; //YYYY-MM-DD
  echo $date.'&lt;br&gt;';
  for ($i = 1; $i &lt; 13; $i++) {
      echo date("Y-m-d H:i:s", strtotime("-$i months", strtotime($date))).'&lt;br&gt;';
  }
  //Func 3
  echo "&lt;br&gt;&lt;br&gt;";
  $dateObj = new DateTime("2014-12-31 23:59:59");
  echo $dateObj-&gt;format("Y-m-d H:i:s").'&lt;br&gt;';
  $dateIntervalObj = new DateInterval("P1M");
  for ($i = 0; $i &lt; 12; $i++) {
      echo $dateObj-&gt;format("Y-m-d H:i:s").'&lt;br&gt;';
      $dateObj-&gt;sub($dateIntervalObj);
  }
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>2014-12-31 23:59:59
  2014-11-30 23:59:59
  2014-10-31 23:59:59
  2014-09-30 23:59:59
  2014-08-31 23:59:59
  2014-07-31 23:59:59
  2014-06-30 23:59:59
  2014-05-31 23:59:59
  2014-04-30 23:59:59
  2014-03-31 23:59:59
  2014-02-28 23:59:59
  2014-01-31 23:59:59
  2013-12-31 23:59:59
  
  
  2014-12-31 23:59:59
  2014-12-01 23:59:59
  2014-10-31 23:59:59
  2014-10-01 23:59:59
  2014-08-31 23:59:59
  2014-07-31 23:59:59
  2014-07-01 23:59:59
  2014-05-31 23:59:59
  2014-05-01 23:59:59
  2014-03-31 23:59:59
  2014-03-03 23:59:59
  2014-01-31 23:59:59
  2013-12-31 23:59:59
  
  
  2014-12-31 23:59:59
  2014-12-31 23:59:59
  2014-12-01 23:59:59
  2014-11-01 23:59:59
  2014-10-01 23:59:59
  2014-09-01 23:59:59
  2014-08-01 23:59:59
  2014-07-01 23:59:59
  2014-06-01 23:59:59
  2014-05-01 23:59:59
  2014-04-01 23:59:59
  2014-03-01 23:59:59
  2014-02-01 23:59:59
  </code></pre>