Get the right timestamp for old log files

Elastic Stack Logstash
56 
input {
  syslog {
    port => 5514
    codec => "json"
  }
  file {
    path => "/var/externallogs_maven/data"
    type => "nexus-log"
    start_position => "beginning"
  }
}
filter {

   grok {

     type => "nexus-log"

     match => [
        "message", "\b\w+\b\s/nexus/content/repositories/(?<repositories>[^/]+)",
        "message", "(?<mytimestamp>%{MONTHDAY}/%{MONTH}/%{YEAR}:%{HOUR}:%{MINUTE}:%{SECOND} %{ISO8601_TIMEZONE})"

      ]
   }
   date{
      match => ["mytimestamp", "dd/MMM/YYYY:HH:mm:ss Z" ]

   }

}
output {


  elasticsearch{
    host => es
    port => 9300
    cluster => "elkjepp"
    protocol => "transport"
 }
 stdout { codec => rubydebug }
}