  We prove that every $(3+1)$-dimensional flat GHMC Minkowski spacetime which
is not a translation spacetime or a Misner spacetime carries a unique foliation
by spacelike hypersurfaces of constant scalar curvature. In otherwords, we
prove that every such spacetime carries a unique time function with isochrones
of constant scalar curvature. Furthermore, this time function is a smooth
submersion.
