  The afterglow of GRB$\,$170817A/GW$\,$170817 was very unusual, slowly rising
as $F_\nu\propto{}t_{\rm{}obs}^{0.8}\nu^{-0.6}$, peaking at
$t_{\rm{obs,pk}}\sim\,150\;$days, and sharply decaying as
$\sim{}t_{\rm{}obs}^{-2.2}$. VLBI observations revealed an unresolved radio
afterglow image whose flux centroid moved superluminally with
$v_{\rm{app}}\approx4c$, clearly indicating that the afterglow was dominated by
a relativistic jet's compact core. Different jet angular structures explained
the afterglow lightcurves: Gaussian and steep power-law profiles with narrow
core angles $\theta_c\lesssim5^\circ$ and larger viewing angles
$\theta_{\rm{}obs}/\theta_c\sim3-5$. However, a top-hat jet (with sharp edges
at $\theta=\theta_0$) was ruled out since it appeared to produce an early flux
rise much steeper than observed. Using 2D relativistic hydrodynamic simulations
we show that the initial steep flux rise is an artifact caused by the
simulation's finite start time, $t_0$, missing its flux contributions from
$t<t_0$ and sometimes "compensated" using an analytic top-hat jet. While an
initially top-hat jet is not very physical, such simulations are particularly
useful at $t_{\rm{}obs}\gtrsim{}t_{\rm{obs,pk}}$ when the afterglow emission is
dominated by the jet's core and becomes insensitive to its exact initial
angular profile if it drops off sharply outside of the core. We demonstrate
that an initially top-hat jet fits GW$\,$170817/GRB$\,$170817A's afterglow
lightcurves and flux centroid motion at $t_{\rm{}obs}\gtrsim{}t_{\rm{obs,pk}}$,
for $\theta_{\rm{}obs}/\theta_0\approx3$ and may also fit the earlier
lightcurves for $\Gamma_0=\Gamma(t_0)\gtrsim10^{2.5}$. We analytically express
the degeneracies between the model parameters, and find a minimal jet energy of
$E_{\rm{}min}\approx5.3\times10^{48}\;$erg and circum-burst medium density of
$n_{\min}\approx5.3\times10^{-6}~{\rm cm}^{-3}$.
