title: Carbon: return a timestamp based on input date string AND timezone
body: <p>Take someone from Europe/Paris (+02:00) for example.</p>
  
  <p>I want to build a function where I pass in a date and time string ('2016-10-6 06:00 PM') and my time zone identifier ('Europe/Paris'). I want to return the corresponding GMT timestamp.</p>
  
  <pre><code>getTimestamp('2016-10-6 06:00 PM', 'Europe/Paris')
  </code></pre>
  
  <p>Here is what I have so far:</p>
  
  <pre><code>public function getTimestamp($string, $time_zone)
  {
      $c = Carbon::parse($string);
      $c-&gt;timezone = new DateTimeZone($time_zone);
      return $c-&gt;timestamp;
  }
  </code></pre>
  
  <p><strong>Problem</strong></p>
  
  <p>If a pass a date and time of <code>2016-10-06 06:00 PM</code> and a time zone of <code>Europe/Paris</code> I get back a UTC timestamp that corresponds to 2016-10-06 06:00 PM GMT.</p>
  
  <p>It should be returning a time of 04:00 GMT because Europe/Paris is two hours ahead.</p>
  
  <p><strong>Question</strong></p>
  
  <p>How do I pass a date and time zone and get back a GMT timestamp with the correct offset.</p>