  In this paper we present a coherent timing analysis of the 401 Hz pulsations
of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019
outburst. Using observations collected with the Neutron Star Interior
Composition Explorer (NICER), we establish the pulsar spin frequency and
orbital phase during its latest epoch. We find that the 2019 outburst shows a
pronounced evolution in pulse phase over the course of the outburst. These
phase shifts are found to correlate with the source flux, and are interpreted
in terms of hot-spot drift on the stellar surface, driven by changes in the
mass accretion rate. Additionally, we find that the long-term evolution of the
pulsar spin frequency shows evidence for a modulation at the Earth's orbital
period, enabling pulsar timing based astrometry of this accreting millisecond
pulsar.
