  We present the X-ray timing and spectral evolution of the Galactic Center
magnetar SGR J1745-2900 for the first ~4 months post-discovery using data
obtained with the Nuclear Spectroscopic Telescope Array (NuSTAR)} and Swift
observatories. Our timing analysis reveals a large increase in the magnetar
spin-down rate by a factor of 2.60 +/- 0.07 over our data span. We further show
that the change in spin evolution was likely coincident with a bright X-ray
burst observed in 2013 June by Swift, and if so, there was no accompanying
discontinuity in the frequency. We find that the source 3-10 keV flux has
declined monotonically by a factor of ~2 over an 80-day period post-outburst
accompanied by a ~20% decrease in the source's blackbody temperature, although
there is evidence for both flux and kT having levelled off. We argue that the
torque variations are likely to be magnetospheric in nature and will dominate
over any dynamical signatures of orbital motion around Sgr A*.
