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 $Delta tapprox1.74;$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 ($E_{rmgamma,iso}approx5times10^{46};$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 jets 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 $Esim10^{48.5}-10^{49.5}~{rm erg}$, and external density $nsim10^{-5}-10^{-1}~{rm cm}^{-3}$ for a $xi_esim 0.1$ non-thermal electron acceleration efficiency.
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