title: MySQL CONVERT_TZ doesn't work for 2 same conversions
body: <p>I got a table with event time logs, and I need to convert 2 columns to a common timezone.</p>
  
  <p>So if I do (convert to different timezones)</p>
  
  <pre><code>        SELECT id
              ,CONVERT_TZ(starting_time, 'UTC', 'Europe/London') AS starting_time
              ,CONVERT_TZ(finishing_time, 'UTC', 'Europe/Madrid') AS finishing_time
          FROM table
  </code></pre>
  
  <p>I got all the times, but if I run</p>
  
  <pre><code>        SELECT id
              ,CONVERT_TZ(starting_time, 'UTC', 'Europe/London') AS starting_time
              ,CONVERT_TZ(finishing_time, 'UTC', 'Europe/London') AS finishing_time
          FROM table
  </code></pre>
  
  <p>All the time data gets <code>NULL</code>.</p>
  
  <p>I loaded my TZ data with the information here <a href="https://dev.mysql.com/doc/refman/5.0/en/time-zone-support.html" rel="nofollow">https://dev.mysql.com/doc/refman/5.0/en/time-zone-support.html</a> from /usr/share/zoneinfo on RHEL 5 using MySQL 5.1.63.</p>