This read-only integer reports the number of seconds elapsed since the bridge's Topology Change Flag last recorded a topology change. An admin uses this to judge how recently the spanning tree topology was disrupted, helping correlate a user-reported outage with an actual recent reconvergence event. For example, a bdgdeviceTimeTopChg value of 30 seconds right after a user reports a brief network hiccup would corroborate that a real topology change occurred around that time.