title: Change column type from character varying to timestamp without time zone
body: <p>In Rails I created a string column called <code>open_time</code>, but then I realized I should use the <code>datetime</code> type. I did:</p>
  <pre><code>change_column :polls, :open_time, :datetime
  </code></pre>
  <p>But it said:</p>
  <blockquote>
  <p>PG::Error: ERROR:  column &quot;open_time&quot; cannot be cast to type timestamp without time zone<br />
  : ALTER TABLE &quot;polls&quot; ALTER COLUMN &quot;open_time&quot; TYPE timestamp</p>
  </blockquote>
  <p>If I just drop the string column and add new datetime column, I will lose the data stored in the string column. Alternatively, in PostgreSQL I can add a column:</p>
  <pre><code>ALTER TABLE polls ADD COLUMN published_time timestamp;
  </code></pre>
  <p>Ant then try to get the data from the string column like:</p>
  <pre><code>UPDATE polls SET published_time = strToTimeStamp(open_time);
  </code></pre>
  <p>Are there any functions I can use as <code>strToTimeStamp</code> to convert <code>character varying</code> type to <code>timestamp without time zone</code>?</p>