title: Add days to DateTime in PHP -- without modifying original
body: <p>How can I add a number of days to a <code>DateTime</code> object without modifying the original. Every question on StackOverflow appears to be about <code>date</code> and not <code>DateTime</code>, and the ones that do mention <code>DateTime</code> talk about modifying the original.</p>
  
  <p>Eg.</p>
  
  <pre><code>$date = new DateTime('2014-12-31');
  $date-&gt;modify('+1 day');
  </code></pre>
  
  <p>But how can you calculate a date several days in advance without modifying the original, so you can write something like:</p>
  
  <pre><code>if($dateTimeNow &gt; ($startDate + $daysOpen days) {
     //
  }
  </code></pre>
  
  <p>I could always just create another <code>DateTime</code> object, but I'd rather do it the above way.</p>