  We present a timing analysis of the 2002 outburst of the accreting
millisecond pulsar SAX J1808.4-3658. A study of the phase delays of the entire
pulse profile shows a behavior that is surprising and difficult to interpret:
superposed to a general trend, a big jump by about 0.2 in phase is visible,
starting at day 14 after the beginning of the outburst. An analysis of the
pulse profile indicates the presence of a significant first harmonic. Studying
the fundamental and the first harmonic separately, we find that the phase
delays of the first harmonic are more regular, with no sign of the jump
observed in the fundamental. The fitting of the phase delays of the first
harmonic with a model which takes into account the observed exponential decay
of the X-ray flux (and therefore of the mass accretion rate onto the neutron
star) gives important information on the torque acting on the neutron star
during the outburst. We find that the source shows spin-up in the first part of
the outburst, while a spin-down dominates at the end. From these results we
derive an estimate of the neutron star magnetic field strength.
