title: Need Relative Format for DateTime in PHP
body: <p>I'm trying to find the nth day of the month in a timestamp like so:</p>
  
  <pre><code>$day = 15;
  
  $date = new DateTime('@' . $timestamp);
  $date-&gt;modify($day . ' day of current month');
  </code></pre>
  
  <p>This generates an error:</p>
  
  <pre><code>Warning: DateTime::modify(): Failed to parse time string (15 day of current month) at position 7
  </code></pre>
  
  <p>I've also tried "day 15 of current month" and that does not work.</p>
  
  <p>How can I modify my "modify" to find the nth day of the current month in the timestamp?</p>