A simple time-delay estimation approach that requires temporarily retuning the existing PID controller is presented. Closed-loop output data are fitted with autoregressive moving average models at two different controller tunings. The time delay is determined by using the feedback-invariance principle that the closed-loop impulse response coefficients do not change inside the time-delay window. This approach requires no intervention to plant operation and does not estimate the open-loop process model but the time delay only. The estimated time delay is particularly suitable for use in control performance monitoring.