title: Append time zone information to a timestamp column
body: <p>My data is stored without a time zone, in IST (changing this is not an option for now).</p>
  
  <pre><code>select 
    some_date as t0,
    some_date AT TIME ZONE 'Asia/Kolkata' as t1, 
    date_add('minute', -330, some_date) AT TIME ZONE 'Asia/Kolkata' as t2
  from my_table limit 1;
  
  t0: 2016-03-27 09:52:33.000    
  t1: 2016-03-27 15:22:33.000 Asia/Kolkata
  t2: 2016-03-27 09:52:33.000 Asia/Kolkata
  </code></pre>
  
  <p>What I want is <code>t2</code>, however the way I've obtained it is poor. Is there a better way to append time zone information to a timestamp column?</p>