diff --git a/lib/logstash/outputs/mongodb.rb b/lib/logstash/outputs/mongodb.rb
index a2f6e6f8d..4c68ad359 100644
--- a/lib/logstash/outputs/mongodb.rb
+++ b/lib/logstash/outputs/mongodb.rb
@@ -52,9 +52,9 @@ class LogStash::Outputs::Mongodb < LogStash::Outputs::Base
       if @isodate
         # the mongodb driver wants time values as a ruby Time object.
         # set the @timestamp value of the document to a ruby Time object, then.
-        document = event.to_hash.merge("@timestamp" => event.ruby_timestamp)
-      else
         document = event.to_hash
+      else
+        document = event.to_hash.merge("@timestamp" => event["@timestamp"].to_json)
       end
       if @generateId
         document['_id'] = BSON::ObjectId.new(nil, event.ruby_timestamp)