This identifies which of the up to 96 completed 15-minute historical buckets a given row of the interval table represents, with interval 1 being the most recently finished quarter hour and higher numbers reaching back in time. This lets an admin reconstruct exactly when in the past 24 hours a problem occurred -- for example if a customer reports the circuit dropped around 3am, the admin can walk the interval numbers backward to find the specific 15-minute bucket that lines up with 3am and inspect its error counts. If the current time is 2pm and interval 8 is being examined, that bucket corresponds to the 15-minute window roughly two hours earlier, since interval N is N quarter-hours before the present.