Date Conversion from GMT to PST

I'm trying to convert the GMT date filed into PST field but it failed to convert the field can anyone help me to resolve it

timestamp=> "2016-03-18 23:59:51"
date {
match => ["timestamp", "YYYY-MM-dd HH:mm:ss" ]
timezone => "PST8PDT"

The date filter always converts to UTC. The timezone option indicates how the source timestamp (in your case the contents of the timestamp field) is to be interpreted.