title: Incorrect timestamp by date processor in elasticsearch filebeat
body: <p>I am getting wrong value in @timestamp field for elasticsearch/filebeat.
  My filebeat pipeline definition</p>
  
  <pre><code>    curl -H 'Content-Type: application/json' -XPUT "logger:9200/_ingest/pipeline/app_log" -d'
      {
        "description" : "Ingest pipeline for Jetty server log",
        "processors" : [
          {
            "grok": {
              "field": "message",
              "patterns": ["%{TIMESTAMP_ISO8601:timestamp} (%{UUID:accessid})? \\[(?&lt;threadname&gt;[^\\]]+)\\] %{LOGLEVEL:level} %{DATA:classname} - %{GREEDYDATA:message}"]
            }
          },
          {
            "date": {
              "field": "timestamp",
              "formats": [ "yyyy-mm-dd H:m:s,SSS" ]
            }
          }
        ],
        "on_failure" : [{
          "set" : {
            "field" : "error.message",
            "value" : "{{ _ingest.on_failure_message }}"
          }
        }]
      }'
  </code></pre>
  
  <p>Simulation result for some sample logger line. ( Using logback configuration )     </p>
  
  <pre><code>curl -H 'Content-Type: application/json' -XPOST "logger:9200/_ingest/pipeline/app_log/_simulate?pretty" -d'
  {
    "docs": [
      {
        "_source": {
          "message": "2018-03-17 22:38:39,079 bab3157d-a11c-4dba-a6d6-c47ae0de2b7f [qtp224100622-174782] INFO  i.n.core.services.cache.CacheBuilder - Key : ChIJrTTTJkdsrjsRXkrYRKRRfd8-seo-localitiesv1 is returned from cache"
  
        }
      },
      {
        "_source": {
          "message": "2017-12-12 01:14:12,079  [qtp224100622-185269] WARN  i.n.m.cache.sdk.RedisCacheProvider - No matching policy: class in.nobroker.core.domain.Token"
        }
      }
    ]
  }' 
  </code></pre>
  
  <p>The result from this simulation:</p>
  
  <blockquote>
    <p>{   "docs" : [
        {
          "doc" : {
            "_index" : "_index",
            "_type" : "_type",
            "_id" : "_id",
            "_source" : {
              "accessid" : "bab3157d-a11c-4dba-a6d6-c47ae0de2b7f",
              "@timestamp" : "2018-01-17T22:38:39.079Z",
              "classname" : " i.n.core.services.cache.CacheBuilder",
              "level" : "INFO",
              "message" : "Key : ChIJrTTTJkdsrjsRXkrYRKRRfd8-seo-localitiesv1 is returned from cache",
              "timestamp" : "2018-03-17 22:38:39,079",
              "threadname" : "qtp224100622-174782"
            },
            "_ingest" : {
              "timestamp" : "2018-03-17T15:35:35.543Z"
            }
          }
        },
        {
          "doc" : {
            "_index" : "_index",
            "_type" : "_type",
            "_id" : "_id",
            "_source" : {
              "@timestamp" : "2017-01-12T01:14:12.079Z",
              "classname" : " i.n.m.cache.sdk.RedisCacheProvider",
              "level" : "WARN",
              "message" : "No matching policy: class in.nobroker.core.domain.Token",
              "timestamp" : "2017-12-12 01:14:12,079",
              "threadname" : "qtp224100622-185269"
            },
            "_ingest" : {
              "timestamp" : "2018-03-17T15:35:35.543Z"
            }
          }
        }   ] }</p>
  </blockquote>
  
  <p>Please notice that @timestamp field is totally different from timestamp field.</p>