Network parameters characterize a network or device, allow one to mathematically insert it into a system, and enable one to calculate or predict system performance. The S parameters may be the most widely measured such parameters; they are readily applied to compute network-port voltages and currents in the frequency domain. However, it is sometimes preferable to compute the voltages and currents in the time domain; for example, when a network-terminating device is nonlinear. This paper provides and demonstrates a numerical method for converting S parameters to time-domain models that can be readily evaluated with ordinary differential equation solvers. No inverse fast Fourier transform or convolution integrals need to be evaluated.