title: Inconsistent DateTime behaviour
body: <p>I would like to get the first monday of the next year and I am getting different output depending on what term I use:</p>
  
  <pre><code>$last1 = new \DateTime('first monday of January 2018');
  /* outputs: 2018-01-01 (correct) */
  
  $last2 = new \DateTime('first monday of January next year');
  /* Outputs 2018-01-02 (wrong) */
  </code></pre>
  
  <p>So having it valid for every year I could do <code>new \DateTime('first monday of January '.date('Y') + 1);</code> but that seems "dirty". Is there something wrong with the second one? Not that THIS years first monday was the second, so I guess it gets parsed wrong?</p>