Difference light curves of SDSS J1001+5027 as obtained by the new difference-smoothing technique introduced in this paper. The curves are shown for the best time-delay estimate found with this technique (top panel, Δt_AB = −118.6 days), and for a wrong time-delay value (bottom panel, Δt_AB = −100.0 days). The difference light curves d_i are shown as colored points. They are smoothed using a kernel of width s = 100 days to compute the f_i (black points). The error bars on the black points show the uncertainty coefficients σ_fi. The points in the difference light curve d_i are color-coded according to the absolute factors of their uncertainties σ_di by which they deviate from f_i. In both panels, light curve A is used as reference, and light curve B is shifted in flux by the same amount.