logstash 如何将如下日志中的时间（非data格式）转换为@timestamp

filter {    

    grok {

        pattern_definitions => { "GROKTIME" => "%{YEAR}.%{MONTHNUM}.%{MONTHDAY} %{HOUR}:%{MINUTE}:%{SECOND}" }

        match => { "message" => "%{GROKTIME:groktime}" }

    }

    date {

        match => [ "groktime", "yyyy.MM.dd HH:mm:ss"]

        remove_field => ["groktime"]

    }

}