Elasticsearch plugin doesn't create file with correct name

Hello.
I'm using the Logstash 5.0 on Ubuntu 14.04.
This is my config

output {
stdout { codec => rubydebug }
elasticsearch { }
}

but index filename is wrong. It is logstash-2006.09.07, but it'd be logstash-2006.04.16.
Any ideas?
Thanks.

That depends on the rest of your config, like grok and date filters.

Sorry Mark but I don't understand. I don't use grok or filters.
Any suggestion is appreciated.
Thanks.

Show us your entire config.

This is my config file...

input {
udp {
port => 9999
codec => netflow { }
type => "netflow"
}
}

output {
elasticsearch {
hosts => ["localhost:9200"]
index => "logstash-netflow5-%{+YYYY.MM.dd}"
}
}

and this is the related file....

yellow open logstash-netflow5-2006.09.06 5 1 17 0 103.9kb 103.9kb

Many thanks.
Regards.
Andrea

Sorry. Solved changing the NetFlow generator tool.
Regards.