title: Convert epoch time in Data Factory to datetime
body: <p>I have the following query.  This query copies the data from <code>Cosmos DB</code> to <code>Azure Data Lake</code>.</p>
  
  <pre><code>select c.Tag
  from c
  where 
  c.data.timestamp &gt;= '@{formatDateTime(addminutes(pipeline().TriggerTime, -15), 'yyyy-MM-ddTHH:mm:ssZ' )}' 
  </code></pre>
  
  <p>However, I've got to use the <code>_ts</code> which is the epoch time when the document was created on the <code>cosmos DB</code> collection instead of <code>c.data.timestamp</code>.  How do I convert epoch time to date time and compare with it with <code>'@{formatDateTime(addminutes(pipeline().TriggerTime, -15), 'yyyy-MM-ddTHH:mm:ssZ' )}'</code> </p>
  
  <p>I have also tried using 
      <code>dateadd( SECOND, c._ts, '1970-1-1' )</code> which clearly isn't supported.</p>