title: elasticsearch sort by date is not working
body: <p>I am having the problem with sorting by date on elasticsearch. As per the documentation provided by elasticsearch <a href="http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/_sorting.html" rel="nofollow">http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/_sorting.html</a> </p>
  
  <p>I used the following code to sort by date </p>
  
  <pre><code>{"sort":{"posted_on":{"order":"desc"}}}
  </code></pre>
  
  <p>The following is the result  </p>
  
  <pre><code>{
  "took": 3,
  "timed_out": false,
  "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
  },
  "hits": {
      "total": 305,
      "max_score": null,
      "hits": [
          {
              "_index": "ads",
              "_type": "ad",
              "_id": "52d6945f1d41c812bf000004",
              "_score": null,
              "_source": {
                  "category": "Cars and Bikes",
                  "description": "Innova 2010(V) DC",
                  "posted_on": "2014-09-04T18:11:57+00:00",
                  "slug_id": "innova-2010-v-dc",
                  "title": "Innova 2010(V) DC"
              },
              "sort": [
                  "57"
              ]
          }
          ,
          {
              "_index": "ads",
              "_type": "ad",
              "_id": "5408accf1d41c8b8f1000001",
              "_score": null,
              "_source": {
                  "category": "",
                  "description": "rrrrrrrrrrfsadsadfsadfsa",
                  "posted_on": "2014-09-04T18:11:57+00:00",
                  "slug_id": "test",
                  "title": "test"
              },
              "sort": [
                  "57"
              ]
          }
          ,
          {
              "_index": "ads",
              "_type": "ad",
              "_id": "52d6945f1d41c812bf000003",
              "_score": null,
              "_source": {
                  "category": "Cars and Bikes",
                  "description": "Indica DLS 2006 for immediate sale",
                  "posted_on": "2015-02-16T12:46:10+00:00",
                  "slug_id": "indica-dls",
                  "title": "Indica DLS"
              },
              "sort": [
                  "46"
              ]
          }
          ,
          {
              "_index": "ads",
              "_type": "ad",
              "_id": "52d694981d41c812d800000a",
              "_score": null,
              "_source": {
                  "category": "Automotive",
                  "description": "MAHINDRA XYLO 2011 MAY Hyderabad registration for immediate sale cash party ",
                  "posted_on": "2015-02-16T12:46:10+00:00",
                  "slug_id": "mahindra-xylo-2011-may-hyderabad-registration",
                  "title": "MAHINDRA XYLO 2011 MAY Hyderabad registration"
              },
              "sort": [
                  "46"
              ]
          }
          ,
          {
              "_index": "ads",
              "_type": "ad",
              "_id": "52d694981d41c812d8000010",
              "_score": null,
              "_source": {
                  "category": "Automotive",
                  "description": "SWIFT LDI 2008 Black,single owner, Hyderabad Registration, Excellent Condition",
                  "posted_on": "2015-02-16T12:46:10+00:00",
                  "slug_id": "swift-ldi-2008-black",
                  "title": "SWIFT LDI 2008 Black"
              },
              "sort": [
                  "46"
              ]
          }
  </code></pre>
  
  <p>Is their anything wrong in query. 
  Thanks, 
  Aashish</p>