title: Timestamp not appearing in Kibana
body: <p>I'm pretty new to Kibana and just set up an instance to look at some ElasticSearch data.</p>
  
  <p>I have one index in Elastic Search, which has a few fields including _timestamp. When I go to the 'Discover' tab and look at my documents, each have the _timestamp field but with a yellow warning next to the field saying "No cached mapping for this field". As a result, I can't seem to sort/filter by time. </p>
  
  <p>When I try and create a new index pattern and click on "Index contains time-based events", the 'Time-field name' dropdown doesn't contain anything. </p>
  
  <p>Is there something else I need to do to get Kibana to recognise the _timestamp field?</p>
  
  <p>I'm using Kibana 4.0. </p>