This records the actual duration, in seconds, of a specific historical 15-minute interval for the pseudowire, which can differ from the nominal 900 seconds if the system clock was adjusted during that window. It's the denominator an engineer needs to correctly convert that interval's stored packet or byte counts into an accurate rate, rather than assuming every interval was exactly 900 seconds long. An interval showing 840 instead of 900 tells you a 60-second clock correction happened during that quarter-hour, and any throughput calculation for that bucket should divide by 840, not 900.