This integer, ranging 1 to 900 seconds, reports how many seconds (including partial seconds) have elapsed since the start of the current mandatory error-measurement period for this SONET/SDH medium. An admin cares because it tells them how much of the current measurement interval has accumulated, letting them judge whether current error counters represent a nearly-complete interval's worth of data or just a few seconds' sample. For example, if this shows 850 out of a 900-second interval, the associated error counts for that period are close to final and can be reasonably compared against thresholds.