PTIDES is based on the semantics of discrete-event (DE) systems, which provides a model of time and concurrency. In the DE model of computation, software components communicate via timestamped events. The event time stamps are a formal part of the model referred to as model time. Model time may or may not bear any relationship to time in the physical world, i.e., real time. In typical DE semantics, each actor processes input events in timestamp order but without constraints on the real time at which events are processed. PTIDES extends DE by establishing the correspondence between model time and real time at sensors, actuators, and network interfaces. In , we use model time to define execution semantics and limit the relationship between model time and real time only to those circumstances where this relationship is needed. Paper  studies the semantic properties important to implementation. It provides an execution model that permits out of order processing of events without sacrificing determinacy and without requiring backtracking.