This records the value of the device's sysUpTime counter at the moment this SONET interface's line status last changed, giving admins a relative timestamp rather than a calendar time; it reads zero if the current state was entered before the last agent restart. Administrators use it to determine how long an interface has held its current line status, which helps distinguish a flapping link from one that has been stable for a long time. A recent value here combined with repeated status-change traps would tell an admin the SONET link is flapping rather than settled.