logstash时间过滤

ue.log，这个ue.log是用的monolog的LogstashFormatter类输出的，所以格式如下了

{"@timestamp":"2016-08-08T00:00:14.003614+08:00","@source":"i-lz0rh3sb","@fields":{"channel":"user_events","level":200,"ctxt_user_id":96836,"ctxt_event":"inst","ctxt_time":1470575487925,"ctxt_name":"种子搜索神器","ctxt_package":"com.yyojjjd.vdio","ctxt_version":"12","ctxt_updated_at":"2016-08-08 00:10:14","ctxt_created_at":"2016-08-08 00:00:14","ctxt_id":5174891},"@message":"user_events","@tags":["user_events"],"@type":"user_events"}
....
....

其中有个字段叫ctxt_created_at的时间字段，格式如："ctxt_created_at":"2016-08-08 00:00:14"

logstash的output的输出的时候，我想新增一个字段保存这个ctxt_created_at的年月日，格式如："ctxt_created_at_format:"2016-08-08"，问下各位，这个怎么实现？我用filter的date插件过滤不出来

配置文件

input {
     file {
                path => "C:/Users/Administrator/Desktop/datas/ue.log"
                type => "user_events"
                start_position => "beginning"
                codec => "json"
            }
}

filter {

    if [type] == 'user_events' {
         
          
          
    }

}

output {

 stdout {
        codec => rubydebug
    }

   
}