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This paper presents a time-domain method for the calculation of the horizontal electric field at the earth surface due to lightning, when the earth structure is made up of two parallel horizontal layers of finitely conducting earth. The method is based on the assessment of the electric field at the surface considering the earth as homogeneous and with the electrical parameters of the first layer. The contribution of the second layer is considered by computing the waves reflected from the boundary between the layers, after they have propagated through the first layer. The paper also shows that the displacement currents could be neglected during the wave reflection, which leads to a simple expression for the reflection coefficient. The results from the method presented in this paper agree with results calculated using a known frequency-domain expression.