TimeStampIndex is a read-only Unsigned32 holding the History time-stamp for this row, expressed as UTC time measured in seconds since midnight, 1 January 1970. Since each row of this table is one historical sample containing a second, parallel set of raw frame and byte traffic counters plus hardware/software error counts for that IP host, this index is what lets an admin retrieve the reading for a particular collection interval instead of only ever seeing the latest value, and — since this isb table duplicates the row layout of the MIB's ilb table but is sampled and indexed independently — a given host can show different collection intervals in isb than it does in ilb. Two rows sharing the same IP host but different TimeStampIndex values represent two separate sampling intervals, so comparing them across time shows the trend in this table's own data — a second, parallel set of raw frame and byte traffic counters plus hardware/software error counts for that IP host.