This flags whether a specific historical 15-minute bucket's error counts are trustworthy, mirroring the CurrentDay validity flag but scoped to one of the 96 stored intervals rather than the whole day. An admin scanning back through interval history to find the moment a fault started needs this flag to rule out false negatives -- an interval showing zero errors but flagged notValid might simply have occurred during a card reset, not an actually clean period. If the line card rebooted at 5:15am, the interval covering 5:00-5:15am would likely read notValid(1) even though its raw counters show zero, since the measurement was interrupted mid-bucket.