We report on the commissioning of an ultrafast timing diagnostic for measuring a time-offset signal between two different synchronized ultrashort light pulses. The method is based on sum-frequency generation in a nonlinear crystal. The setup is similar to an auto/cross-correlator setup. In this case, one of the beams is a relatively weak diagnostic beam of visible light from a bending magnet at FemtoMAX (10 6 photons pulse −1 in a 0.2 mm high and 5 mm wide beam) while the other is a relatively intense laser beam (200 µJ pulse −1 ) derived from the same laser that is used to pump the sample in pump/probe experiments. This enables online monitoring of the relative timing of a linear accelerator-based, short-pulse, hard X-ray source and a synchronized visible laser. We show that for a <50 fs full width at half-maximum (FWHM) light pulse from the accelerator and a 50 fs (FWHM) long laser pulse, we can determine the relative timing of the two pulses with an accuracy below 30 fs in a time interval of 4 ps. The advantages and limitations of the method are discussed.