This object is the sysUpTime at the most recent change to either the source-count address or its enable flag. This is what tells an admin, or a monitoring tool, that the source-filtered counter below was reset or retargeted at a specific point in time, so any earlier count values should not be compared directly against counts collected after this timestamp. Seeing this timestamp advance between two polls explains why srpMACCountPktsBySource suddenly shows a much smaller number than before; the filter was retargeted to a different source address, not that traffic actually dropped.