filter.conf

if [type] == "log_type" {
              grok {
                    patterns_dir => "/path/patterns"
                    match => { "message" => "%{my_custom_defined}" }
              }
              date {
                    match => [ "timestamp", "%{TIMESTAMP_ISO8601}" ]
              }
        }
..
.

where my log starts with: 2014-03-11T04:54:08.876941Z ..