Packet-Original-Timestamp

Hello Everyone I have a NAS which is not sending Event-Timestamp in accounting messages. I wondered if I could create it in unlang by subtracting Acct-Delay-Time from Packet-Original-Timestamp. However, when I put a reference to Packet-Original-Timestamp in the acconting setion I get the following error :- Reference "${Packet-Original-Timestamp}" not found Am I doing something wrong?