title: Logstash date format grok pattern
body: <p>I have a log file which has a date time in 'yyyyMMdd_HHmmss_SSS' format. I am successful in parsing this with _ as delimiter and getting as 3 different text field in ES. But I need this to be converted as ISO_8601 so I can query and visualize the data by date or by hour or by minute. </p>