A time-compensated monochromator for ultrashort high-order harmonics in the EUV and soft X-ray regions is presented. The system consists of two grazing-incidence toroidal mirrors used, respectively, as collimating and refocusing elements and of two multilayer normal-incidence plane mirrors illuminated in parallel light that rotate along a vertical axis to select the operative wavelength, but remain parallel to guarantee the constant direction of the exit beam. This is performed by simultaneously rotating and translating one of the two mirrors along an axis parallel to the exit direction. The pulse time duration is not altered up to few femtoseconds.