Passive photonic integrated circuits (PICs) can be easily characterized in the frequency-domain, but their accurate time-domain performance evaluation is a hurdle for systemlevel designers, especially when dealing with resonant circuits having highly dispersive behavior, such as ring resonators. In this paper, a new equivalent circuit modeling and simulation approach is proposed, based on the Complex Vector Fitting algorithm, able to perform accurate and robust time-domain simulations of passive PICs directly in standard SPICE simulators. The proposed modeling technique starts from scattering parameters of passive PICs, and is able to capture linear and high order dispersion, backscattering, and wavelength dependent effects. Considering the different nature of optical and electronic signals, a novel concept of equivalent voltage and current for optical waveguides is proposed to simplify the optical to electronic ports conversion and to make it possible to connect and terminate the equivalent circuit models as needed in SPICE simulators, natively supporting bidirectional signal propagation in a waveguide. This work provides a precise and reliable solution to evaluate timedomain characteristics of passive PICs and to access any internal nodes within a circuit, such as the signals inside a ring resonator. Three examples of time-domain simulations of passive PICs in commercial SPICE simulators are presented to demonstrate the flexibility and advantages of the proposed technique.