date filter: omitting date results in 1970-01-01

cite@helena:~$ cat /tmp/date.conf
input { stdin { } }
output { stdout

{ codec => rubydebug }

}
filter {
grok { match => ["message", "%

{TIME:timeonly}

%

{GREEDYDATA:remainder}

"] }
date

{ match => ["timeonly", "HH:mm:ss"] remove_field => ["timeonly"] }

}
cite@helena:~$ echo 11:38:52 hello world | java -jar Downloads/logstash-1.3.3-flatjar.jar agent -f /tmp/date.conf

{ "message" => "11:38:52 hello world", "@version" => "1", "@timestamp" => "1970-01-01T11:38:52.000+01:00", "host" => "helena", "remainder" => "hello world" }

I'd say that's an unexpected result. Couldn't we simply assume the current day?

Thanks
Stefan