title: Can't create an index on a jsonb field with date
body: <p>I have a jsonb column <code>event</code> in a table. I am trying to create an index</p>
  
  <pre><code>CREATE INDEX ON table(((events-&gt;'START'-&gt;&gt; 'date')::timestamp AT TIME ZONE 'PST'));
  </code></pre>
  
  <p>but it is throwing error <code>functions in index expression must be marked IMMUTABLE</code></p>
  
  <p>After passing it a timezone it should make it immutable, but I am not sure why it's still throwing an error.</p>