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 breakdown of ICMP message types seen 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 isi table duplicates the row layout of the MIB's ili table but is sampled and indexed independently — a given host can show different collection intervals in isi than it does in ili. 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 breakdown of ICMP message types seen for that IP host.