Aggregates all lines messages between 2 date using multiline filter

Elastic Stack Logstash
1

I have a log like this:

2015-06-26 23:00:48,935  foo                  as "date1"
whatever lines not start with date
...
2015-06-16 18:46:16,136 bar                   as "date2"

I'm trying to get all lines between date1(include) and date2(exclude). So I use "multiline" filter:

filter {
    grok {
        match => {"message" => "%{TIMESTAMP_ISO8601:[@metadata][tt]} %{GREEDYDATA:[@metadata][dd]}"}
    }
    if "_grokparsefailure" not in [tags] {
         date {
             match => ["[@metadata][tt]", "yyyy-MM-dd HH:mm:ss,SSS"]
         }
     } else { # indicates this line should belong to previous message
         multiline {
             pattern => ".*"
             what => "previous"
         }
     }
 }

But it just generates 3 messages:

m1: 2015-06-26 23:00:48,935  foo
m2: 2015-06-16 18:46:16,136 bar
m3: whatever lines not start with date
    ...

What's wrong with the filter config? How to make it right? Thx in advance!

2

Answering my own question after I solved it:

filter {
    multiline {
        pattern => "^%{TIMESTAMP_ISO8601}"
        negate => true
        what => "previous"
    }
    grok {
        match => {"message" => "%{TIMESTAMP_ISO8601:[@metadata][tt]} %{GREEDYDATA:[@metadata][dd]}"}
    }
    date {
        match => ["[@metadata][tt]", "yyyy-MM-dd HH:mm:ss,SSS"]
    }
}