This reports the value of sysUpTime at the moment this encrypted connection was established or most recently re-established, per its description - essentially a timestamp anchored to device uptime rather than wall-clock time. An admin uses it to calculate how long the current encryption session has actually been up, or to correlate a connection's establishment moment against other uptime-based events like an interface flap or a key renegotiation. If sysUpTime is currently 500000 (in hundredths of a second) and this object reads 480000, the connection has been up for roughly 200 seconds.