Extract timestamp from JSON and use it as the logstash event timestamp
I think you may have misunderstood how the date filter (there don't appear to be docs for 1.3.1) works, it takes a field, and parses it against a pattern you set and uses that match to set the value of the @timestamp field (by default). Since you are taking the @timestamp field, matching it against a pattern and then trying to set the @timestamp field with it, what you're doing is redundant.
If you have a different timestamp field you want to take the date from I expect it would probably work, as long as you were using the right pattern for that field.
I would also recommend you update to 1.4.2 as the older version you are running might be buggy.