Use the date filter to parse a string and convert it to an ISO8601 timestamp (usually stored in the @timestamp). No Ruby filter is necessary for this.
Note that what happens on the Elasticsearch side isn't really up to Logstash. Logstash sends JSON documents to ES, and scalars in JSON documents are strings, numbers, or booleans. To have a timestamp stored as a date in ES you need to change the mapping. This is done by default for the @timestamp field.