Logstash s3 input plugin specify dynamic prefix

Hi.
I would like to use a dynamicaly changing path in prefix based on the current month and year, prefix => "elasticloadbalancing/region/YYYY/MM"

input {
  s3 {
    access_key_id => ""
    secret_access_key => "38/"
    bucket => "elb"
    prefix => "elasticloadbalancing/region/YYYY/MM"
    type => "aws-elb"
  }
}

Is it somehow possible to achieve?
Thanks!