Is there a way to include a Timestamp of the Last Update by End User?

While the standard calculated attribute f\can provide a timestamp for updates by agents/admins like below:

IF ([Comment present]=TRUE 
AND [Comment public]=TRUE
AND ([Updater role]="Agent" OR [Updater role]="Admin"))
THEN [Update - Timestamp] ENDIF

When I try this with "End-user" or "End User"

IF ([Comment present]=TRUE 
AND [Comment public]=TRUE
AND ([Updater role]="End User"
THEN [Update - Timestamp] ENDIF

IF ([Comment present]=TRUE 
AND [Comment public]=TRUE
AND ([Updater role]="End-User"
THEN [Update - Timestamp] ENDIF

No values are returned. 

This would be great for our customers since they could see when someone other than the requester has last updated the ticket. 

While there is a "Time - Ticket last updated by requester - Date" we need to pull all end users since CC'd users also contribute to the conversation.