Ah okay, now I think I understood.
Right, you know better about the timestamp format.
The date filter in logstash ALWAYS converts to the UTC time. Now what you want, is to have a field which has the timestamp in your own timezone. This is not possible with the date filter and your only option is to use the ruby filter, as you have already done.
If this is what you want, you can also check my other reply about the same issue.