title: How to aggregate by year-month-day on a different timezone
body: <p>I have a MongoDB whom store the date objects in UTC. Well, I want to perform aggregation by year,month day in a different timezone (CET).</p>
  
  <p>doing this, works fine for UTC:</p>
  
  <pre><code>    BasicDBObject group_id = new BasicDBObject("_id", new BasicDBObject("year", new BasicDBObject("$year", "$tDate")).
                  append("month", new BasicDBObject("$month", "$tDate")).
                  append("day", new BasicDBObject("$dayOfMonth", "$tDate")).
                  append("customer", "$customer"));
  
      BasicDBObject groupFields = group_id.
              append("eventCnt", new BasicDBObject("$sum", "$eventCnt")); 
  
      BasicDBObject group = new BasicDBObject("$group", groupFields);
  </code></pre>
  
  <p>or, if you use the command line (not tested, I only tested the java version):</p>
  
  <pre><code>{
      $group: {
          _id: {
              "year": {
                  "$year", "$tDate"
              },
              "month": {
                  "$month", "$tDate"
              },
              "day": {
                  "$dayOfMonth", "$tDate"
              },
              "customer": "$customer"
          },
          "eventCount": {
              "$sum": "$eventCount"
          }
      }
  }
  </code></pre>
  
  <p>How do I convert these dates into CET inside the aggregation framework?</p>
  
  <p>For example '2013-09-16 23:45:00 UTC'  is '2013-09-17 00:45:00 CET', this is a different day.</p>