  We propose a method for a straightforward characterization of the temporal
shape of femtosecond pulses in the extreme-ultraviolet/soft X-ray spectral
region. The approach is based on the presence of a significant linear frequency
chirp in the pulse. This allows to establish an homothetic relation between the
pulse spectrum and its temporal profile. The theoretical approach is
reminiscent of the one employed by Fraunhofer for describing far-field
diffraction. As an application, we consider the case of a seeded free-electron
laser (FEL). Theory is successfully benchmarked with numerical simulations and
with experimental data collected on the FERMI@Elettra FEL. The proposed method
provides FEL users with an on-line, shot-to-shot spectro-temporal diagnostic
for time-resolved experiments.
