Logstash nmap codec @timestamp match

I am using Logstash to input NMAP scan data into Elasticsearch.

input{
http{
host => "127.0.0.1"
port => 8000
codec => nmap
}
}

filter{
date{
match => [ "start_time", "ISO8601"]
target => "@timestamp"
}
}

output{
stdout{codec => rubydebug}
}

Using the above config file, @timestamp will not update to match the start_time.

{
      "start_time" => "2016-02-12T19:40:33.000Z",
        "end_time" => "2016-02-12T21:19:44.000Z",
       "addresses" => [
        [0] {
            :type => "ipv4",
            :addr => "192.168.30.224"
        }
    ],
         "address" => "192.168.30.224",
              "ip" => "192.168.30.224",
            "ipv4" => "192.168.30.224",
            "ipv6" => nil,
             "mac" => nil,
          "status" => {
         "state" => "up",
        "reason" => "timestamp-reply"
    },
        "hostname" => nil,
          "uptime" => nil,
              "os" => nil,
       "arguments" => "/usr/bin/nmap -T5 -oX /home/NMAP/20160212_1200__INT_DEV.xml -oG /home/NMAP/20160212_1200__INT_DEV.gnmap -iL /home/NMAP/ip-list.txt",
         "version" => "6.47",
         "scan_id" => "bc9e8106-4536-412e-b44a-1de67451318d",
            "type" => "nmap_port",
            "port" => {
          "number" => 445,
          "reason" => "syn-ack",
        "protocol" => "tcp",
         "service" => {
                          "name" => "microsoft-ds",
                           "ssl" => false,
                      "protocol" => nil,
                       "product" => nil,
                      "hostname" => nil,
                   "device_type" => nil,
            "fingerprint_method" => "table",
                   "fingerprint" => nil,
                    "confidence" => 3
        },
           "state" => "open"
    },
    "scan_host_id" => "bc9e8106-4536-412e-b44a-1de67451318d-h663",
              "id" => "bc9e8106-4536-412e-b44a-1de67451318d-h663-p2",
        "@version" => "1",
      "@timestamp" => "2016-02-15T22:46:50.986Z",

Any ideas? Im sure im missing something. The one example i can find of using the nmap codec is on elastic searches site but does not do the timestamp match.

Thanks