  We present temporal analysis of the three outbursts of the X-ray millisecond
pulsar SAX J1808.4-3658 that occurred in 1998, 2000 and 2002. With a technique
that uses the chi^2 obtained with an epoch folding search to discriminate
between different possible orbital solutions, we find an unique solution valid
over the whole five years period for which high temporal resolution data are
available. We revise the estimate of the orbital period, P_orb =7249.1569(1) s
and reduce the corresponding error by one order of magnitude with respect to
that previously reported. Moreover we report the first constraint on the
orbital period derivative, -6.6 x 10^-12 < Pdot < +0.8 x 10^-12 s/s. These
values allow us to produce, via a folding technique, pulse profiles at any
given time. Analysis of these profiles shows that the pulse shape is clearly
asymmetric in 2002, occasionally showing a secondary peak at about 145 degrees
from the main pulse, which is quite different from the almost sinusoidal shape
reported at the beginning of the 1998 outburst.
