hi @tomer,
I think you'll find better luck in the Beats forum indeed: cf. Changing a term in a json to become the timestamp from filebeat
One thing you can consider in Kibana (but I wouldn't recommend it), is using a scripted field to convert that time-stamp to a date. To get started, see https://www.elastic.co/guide/en/kibana/current/scripted-fields.html. That way, you'll convert the time-stamp on-the-fly at query-time instead of index-time.