title: Add 90 days to a Y-m-d H:i:s formatted date, in Unix Timestamp format
body: <p>How can I add 90 days to a <code>Y-m-d H:i:s</code> formatted date that's pre-defined and convert it to a Unix Timestamp so I can send it as an expiration argument for <code>setcookie()</code>?</p>
  <p>I've tried the following:</p>
  <pre><code>var_dump(strtotime('+90 days', '1994-02-21 08:01:13'));
  </code></pre>
  <p>Which outputs:</p>
  <blockquote>
  <p><strong>Notice:</strong> A non well formed numeric value encountered in [file path] on line <strong>3</strong>.</p>
  <p>int(7777994)</p>
  </blockquote>