Hi,
After some research I found the confirmation that when using the elasticsearch output plugin and using the date for the index name, it automatically uses the '@timestamp' field. I was also interested in finding in the source code (https://github.com/logstash-plugins/logstash-output-elasticsearch) where that happens.
I have very little experience with ruby and can't seem to find that specific part of the code, therefore I'd like ask for some help in finding it.