Many types of circuits, e.g., switched-capacitor filters, mixers, voltage-controlled oscillators (VCOs), etc., contain waveforms that change
at widely disparate rates. We show that it is useful to reformulate such systems in terms of multiple time variables, thereby transforming
differential-algebraic equations (DAEs) to partial differential equations (PDEs) which can be easier to solve numerically.  We present multi-time
formulations for non-autonomous systems (like mixers) and also for forced autonomous systems (like VCOs). If time permits, we will also describe how multitime formulations are useful for automatic macromodelling of time-varying systems..