title: setISODate work that way?
body: <p><strong>I am testing these cases:</strong></p>
  
  <pre><code>$date = new \DateTime();
  echo($date-&gt;format('Y.m.d')) . PHP_EOL;
  
  $date-&gt;setISODate(2018, 1, 1);
  echo($date-&gt;format('Y.m.d')) . PHP_EOL;
  
  $date-&gt;setISODate(2019, 1, 1);
  echo($date-&gt;format('Y.m.d')) . PHP_EOL;
  
  $date-&gt;setISODate(2020, 1, 1);
  echo($date-&gt;format('Y.m.d')) . PHP_EOL;
  </code></pre>
  
  <p><strong>Ouput:</strong></p>
  
  <pre><code>2017.08.17
  2018.01.01
  2018.12.31
  2019.12.30
  </code></pre>
  
  <p><strong>I understood that it would show this:</strong></p>
  
  <pre><code>2017.08.17
  2018.01.01
  2019.01.01
  2020.01.01
  </code></pre>
  
  <p>Why does that happen? </p>
  
  <p>Surely there is something I did not quite understand.</p>