title: mysql CONVERT_TZ inconsistent behavior
body: <p>The following behavior puzzles me. My mysql server is installed on a PC with EEST timezone.
  I want to convert a datetime in mysql to UTC date.</p>
  
  <p>When executing </p>
  
  <pre><code>select CONVERT_TZ('1970-01-01 02:00:01', 'SYSTEM', '+00:00')
  </code></pre>
  
  <p>the reponse is:
  '1970-01-01 00:00:01'</p>
  
  <p>Similarly, when executing </p>
  
  <pre><code>select CONVERT_TZ('1970-01-01 04:00:00', 'SYSTEM', '+00:00')
  </code></pre>
  
  <p>the response is:'1970-01-01 02:00:00'</p>
  
  <p>However, when I basically want the 0 unix time, by executing:</p>
  
  <pre><code>select CONVERT_TZ('1970-01-01 02:00:00', 'SYSTEM', '+00:00')
  </code></pre>
  
  <p>The reponse is again: '1970-01-01 02:00:00'</p>
  
  <p>Why is that? Am I doing something wrong? How can I get the correct value which is '1970-01-01 00:00:00'?</p>