title: Use parts (day, hour...) of @timestamp logstash in other fields
body: <p>I need to add the current day extracted from the @timestamp field in the message field itself.</p>
  
  <p>Example:
      "@timestamp" => "2015-03-20T07:27:35.589Z",</p>
  
  <p>--> I want to add "20" to the message field</p>
  
  <p>Maybe there is some very easy way to do this, without grok...?</p>