title: Incorrect date format in elasticsearch result
body: <p>My document looks as following:</p>
  
  <pre><code>{
     'time'   : &lt;timestamp_with_millis&gt;,
     'amount' : &lt;int&gt;
  }
  </code></pre>
  
  <p>I am doing this query:</p>
  
  <pre><code>{
      "aggs" : {
          "articles_over_time" : {
              "date_histogram" : {
                  "field" : "time",
                  "interval" : "1M",
                  "format" : "yyyy-MM-dd HH:mm:ss" 
              }
          }
      }
  }
  </code></pre>
  
  <p>And I get:</p>
  
  <pre><code>"articles_over_time": {
        "buckets": [
          {
            "key_as_string": "yyyy-MM-dd HH:mm:ss1451606400000",
            "key": 1451606400000,
            "doc_count": 63
          }
        ]
      }
  </code></pre>
  
  <p>Can somebody explain me why the key is at the end of <code>key_as_string</code> value and the timestamp is not formatted?</p>