Knowing that short-time propagators for Fokker-Planck equations are Gaussian, and based on a path-sum formulation, an efficient and simple numerical method is presented to solve the initial-value problem for electron kinetics during rf heating and current drive. The formulation is thoroughly presented and discussed, its advantages are stressed, and general, practical criteria for its implementation are derived regarding the time step and grid spacing. The new approach is illustrated and validated by solving the one-dimensional model for lower-hybrid current drive, which has a well-known steady-state analytical solution.