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This paper reports an evaluation of the accuracy of the discrete Green's function (DGF) formulation of the finite-difference time-domain (FDTD) method. Recently, the closed-form expression for the DGF and its efficient numerical implementation were presented, which facilitates applications of the DGF in FDTD simulations of radiation and scattering problems. So far, the accuracy of the DGF formulation of the FDTD method has been rather marginally treated in the literature. Moreover, although windowing has been reported as an efficient method of DGF waveform truncation and a remedy for stability issues, the accuracy and usability of this technique have not yet been fully evaluated. In this paper, previously unrevealed accuracy limitations of the DGF formulation of the FDTD method are demonstrated in several numerical tests. Specifically, the truncation errors are compared for the most frequently applied windowing functions, with the best performance shown for the Hann's window.