User can define a column that computes the duration between two dates(<date column>, [date column]). Something like:
duration(firstEventTime) /[date] defaults to now()/
duration(firstEventTime, ackTime) /* would set col head to NOC Response Time */
Perhaps the the technique allows the user to pick a "from" column, such as ackTime, then the select an "aggregation function" called duration that then allows you to pick the "to" column with default as now().