  The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc)
elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the
first gravitational wave (GW) detection of a binary neutron-star (NS-NS)
merger. It was followed by optical, IR, and UV emission from half a day up to
weeks after the event, as well as late time X-ray and radio emission. The early
UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of
radioactive-decay powered kilonova-like emission. Comparison to kilonova models
favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is
also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp
signal and the prompt GRB onset. However, the late onset of the X-ray
(8.9$\;$days) and radio (16.4$\;$days) emission, together with the low
isotropic equivalent $\gamma$-ray energy output
($E_{\rm\gamma,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from
a narrow relativistic jet viewed off-axis. Here we set up a general framework
for off-axis GRB jet afterglow emission, comparing analytic and numerical
approaches, and showing their general predictions for short-hard GRBs that
accompany binary NS mergers. The prompt GRB emission suggests a viewing angle
well outside the jet's core, and we compare the afterglow lightcurves expected
in such a case to the X-ray to radio emission from GRB 170817A. We fit an
afterglow off-axis jet model to the X-ray and radio data and find that the
observations are explained by a viewing angle $\theta_{\rm
obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm
erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a
$\xi_e\sim 0.1$ non-thermal electron acceleration efficiency.
