With the technical progress of radio-frequency setups , high frequency quantum transport experiments have moved from theory to the lab. So far the standard theoretical approach used to treat such problems numerically-known as Keldysh or NEGF (Non Equilibrium Green's Functions) formalism-has not been very successful mainly because of a prohibitive computational cost. We propose a reformulation of the non-equilibrium Green's function technique in terms of the electronic wave functions of the system in an energy-time representation. The numerical algorithm we obtain scales now linearly with the simulated time and the volume of the system, and makes simulation of systems with 10$^5$ − 10$^6$ atoms/sites feasible. We illustrate our method with the propagation and spreading of a charge pulse in the quantum Hall regime. We identify a classical and a quantum regime for the spreading, depending on the number of particles contained in the pulse. This numerical experiment is the condensed matter analogue to the spreading of a Gaussian wavepacket discussed in quantum mechanics textbooks.