How to replace @timestamp with actual log time

Elastic Stack Logstash
15

how to parse time stamp , even though if there grokparse failure ? bcoz my all log messages are not same, below is example

"@timestamp" => 2018-09-21T06:44:31.018Z,
  "@version" => "1",
      "tags" => [
    [0] "_grokparsefailure"
],
      "host" => "node1",
   "message" => "2018-09-20 12:19:42 [INFO] from application in pool-3-thread-5 - Authenticate DQL processing time for transactionId : 6064824338348622_node1 documentType : License merchant : 70214f84- is 376 msec"