As this thread explains (https://stackoverflow.com/a/11141819/474002): Timestamp is not part of the regular messages. Therefore you cannot extract it.
I think you could send a custom packet (use PacketExtension) to extend your message packet to include the timestamp information.
Message message = new Message();
DefaultPacketExtension yourExt = new DefaultPacketExtension("your_ext", "com:your:ext");
yourExt.setValue("timestamp", YOUR_TIMESTAMP);
messgae.addExtension(yourExt);