In this talk, we describe excitation of time-varying dipolar particles (time-modulated meta-atoms) by external time-varying fields from a nonstationary and causal perspective. For a time-harmonic excitation, we introduce a complex-valued function, called temporal complex polarizability, and use this function to study a classical electron through the equation of motion whose damping coefficient and natural frequency are changing in time. We theoretically derive the temporal complex permittivity corresponding to time-varying media formed by timemodulated meta-atoms and explicitly show the differences with the conventional macroscopic Drude-Lorentz model.