Time evolution of the four-velocity γβ of the hypermassive magnetar wind, calculated assuming that the wind mass-loss rate evolves as ﹩\dot{M}\propto {L}_{\nu }^{5/3}{\epsilon }_{\nu }^{10/3}﹩, where the cooling evolution of the neutrino luminosity L_ν(t) and mean neutrino energy ϵ_ν(t) are taken from the 2.0M_⊙ proto-NS model of Pons et al. (1999). We separately show a case in which the magnetic field strength is held constant in time at B ≈ 2 × 10¹⁴ G (black solid line) and one in which the field strength grows exponentially on a timescale of 0.5 s, (dashed blue line). In both cases, the wind velocity over the first ∼0.1–1 s are in the range ∼0.1–0.3 c needed to explain the blue KN ejecta, but in the latter case the wind becomes transrelativistic (γβ ≳ 1) by the onset of GRB 170817A, in which case relativistic breakout of internal shocks within the wind (Nakar & Sari 2012) could serve as a potential origin for the gamma-rays.