  We present a timing analysis of the 2015 outburst of the accreting
millisecond X-ray pulsar SAX J1808.4-3658, using non-simultaneous XMM-Newton
and NuStar observations. We estimate the pulsar spin frequency and update the
system orbital solution. Combining the average spin frequency from the previous
observed, we confirm the long-term spin down at an average rate
$\dot{\nu}_{\text{SD}}=1.5(2)\times 10^{-15}$ Hz s$^{-1}$. We also discuss
possible corrections to the spin down rate accounting for mass accretion onto
the compact object when the system is X-ray active. Finally, combining the
updated ephemerides with those of the previous outbursts, we find a long-term
orbital evolution compatible with a binary expansion at a mean rate
$\dot{P}_{orb}=3.6(4)\times 10^{-12}$ s s$^{-1}$, in agreement with previously
reported values. This fast evolution is incompatible with an evolution driven
by angular momentum losses caused by gravitational radiation under the
hypothesis of conservative mass transfer. We discuss the observed orbital
expansion in terms of non-conservative mass transfer and gravitational
quadrupole coupling mechanism. We find that the latter can explain, under
certain conditions, small fluctuations (of the order of few seconds) of the
orbital period around a global parabolic trend. At the same time, a
non-conservative mass transfer is required to explain the observed fast orbital
evolution, which likely reflects ejection of a large fraction of mass from the
inner Lagrangian point caused by the irradiation of the donor by the
magneto-dipole rotator during quiescence (radio-ejection model). This strong
outflow may power tidal dissipation in the companion star and be responsible of
the gravitational quadrupole change oscillations.
