We present the design and characterization of a double-configuration grating monochromator for the spectral selection of extreme-ultraviolet ultrafast pulses. Two grating geometries are joined in an instrument with two interchangeable diffracting stages, both used at grazing incidence: one with the gratings in the off-plane mount (OPM), the other in the classical diffraction mount (CDM). The use of two stages gives great flexibility: the OPM stage is used for sub-50 fs time response and low spectral resolution, while the CDM stage is for 100-200 fs time response and high spectral resolution. The monochromator spectral and temporal performances have been experimentally demonstrated on a high-order laser-harmonics beam line.