I did not find the exact use case in the forum, but searching on it and with some tries, here is my solution :
match => [ "startTime", "dd/MM/YYYY HH:mm:ss.SSS" ]
timezone => "Europe/Paris"
target => "startTime"
init => "require 'time'"
code => "
durationInSeconds = event.get('durationMs').to_f / 1000;
#Addition of duration must be in seconds here with a float number eventually for ms
timeVal = Time.iso8601(event.get('startTime').to_s) + durationInSeconds
#Converts to logstash object of timestamp :
It works also with logstash 2.3 with the correct event manipulation as an array instead of getter and setter.