title: date_trunc in hive is working incorrectly
body: <p>I am running below query:</p>
  
  <pre><code>select a.event_date,
      date_format(date_trunc('month', a.event_date), '%m/%d/%Y') as date
  from monthly_test_table a
  order by 1;
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code> 2017-09-15 | 09/01/2017
   2017-10-01 | 09/30/2017
   2017-11-01 | 11/01/2017
  </code></pre>
  
  <p>Can anyone tell me why for date "2017-10-01" it is showing me date as "09/30/2017" after using date_trunc.</p>
  
  <p>Thanks in Advance...!</p>