Abstract: A method and system are provided for determining a guard band voltage differential for testing a microprocessor. The guard band voltage differential approximates microprocessor circuit propagation delay degradation expected to occur over the life of the microprocessor. The system and method are performed by first partitioning a microprocessor into a plurality of cones of n circuit level models. Timing simulation data and degradation data are created to represent, respectively, the timing operation for each of the circuit level model circuit paths, and the hot-electron effects on propagation delay degradation for each of the circuit level models. Propagation delay is identified using this data for each of the circuit paths for the circuit level models at times corresponding to the beginning-of-life and end-of-life of the microprocessor. Propagation delay degradation is calculated as the difference between the propagation delay at these times.