title: Qt 5.5 QDateTime::addSec function has returned a wrong value in a last day of a month if add 12 hours (43200 secs)
body: <p>Qt 5.5 QDateTime::addSec function has returned a wrong value in a last day of a month if add 12 hours (43200 secs).</p>
  
  <p>I's got the following result today 30 november 2015:</p>
  
  <p>1.</p>
  
  <pre><code>QDateTime::currentDateTimeUtc().addSecs(43200);
  </code></pre>
  
  <p>output (run time - 2015-11-30T10:05:50):</p>
  
  <pre><code>date - Mon Nov 30 10:05:50 2015 GMT
  </code></pre>
  
  <p>2.</p>
  
  <pre><code>QDateTime::currentDateTimeUtc().addSecs(21600);
  </code></pre>
  
  <p>output (run time - 2015-11-30T10:08:12): </p>
  
  <pre><code>date - Mon Nov 30 16:08:12 2015 GMT
  </code></pre>
  
  <p>3.</p>
  
  <pre><code>QDateTime::currentDateTimeUtc().addDays(1).addSecs(-43200);
  </code></pre>
  
  <p>output (run time - 2015-11-30T10:11:08): </p>
  
  <pre><code>date - Tue Dec 1 10:11:08 2015 GMT
  </code></pre>
  
  <p>I want to add 12 hours and get 2015-11-30T22:11:08
  How to avoid this situation? Is it a bug in Qt or I do something wrong?</p>