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The contribution deals with a new time-domain near-to-far-field (NFF) transformation which is particularly suited for the finite-difference time-domain (FDTD) method. The far field is derived from the tangential field components on a surface enclosing the scatterer by employing a time-domain spherical-multipole representation. The necessary time-domain convolution is performed as "on the fly," in parallel with the FDTD time-stepping algorithm. The efficiency of the method is improved by incorporating a temporal linear interpolation of the near-field data. With the once obtained multipole amplitudes an analytic series representation of the time-domain far field is achieved, which allows a physical interpretability of the result. Moreover, this expansion serves as an ideal basis for a systematic and efficient post processing. The proposed technique may also be useful for other numerical and for asymptotic methods.