The design of cyber-physical systems requires the use of mixed continuous time and discrete time models. Current modelling tools have problems with time transformations (such as a time delay) or multi-rate systems. We will present a novel approach that implements signals as functions of time, directly corresponding to their mathematical representation. This enables an exact implementation of time transformations and as an additional advantage enables local control over time. A representation of components and signals in both domains is provided, together with composition operators to allow the specification of signal flow diagrams.