If the circuit is enabled, this is the value of sysUpTime when ciiCircAdminState most recently entered the state 'on'; if the circuit is not on, it is the sysUpTime when it last entered on, or 0 if it has never been on. This gives an admin a precise timestamp for when this specific circuit was last brought into service, the field to check when determining how long a currently-down circuit has actually been down. For example, a circuit showing ciiCircLastUpTime of 0 despite being administratively enabled tells the admin the circuit has never successfully come up even once since being configured, pointing to a configuration or physical-layer problem rather than a recent flap.