Records the timestamp of the alarm's most recent state change -- such as a severity update or new additional text -- falling back to the original raise time (matching tfAlarmOrigTime) if the alarm has never changed state. An admin uses this to see how recently an alarm was last updated, distinguishing a freshly escalated alarm from one that's been sitting unchanged for a long time. For example, if tfAlarmTime is much later than tfAlarmOrigTime, it tells the admin the alarm's severity or details were recently revised rather than newly raised.