  The directed flow of particles emitted from the fireball created in a
heavy-ion collision is shown to be a very sensitive measure of the pressure
equilibration in the first 1 fm/c of the evolution. Performing a 3+1
dimensional relativistic hydrodynamic calculation with nonequilibrated
longitudinal and transverse pressures, we show that the directed flow is
strongly reduced if the pressure imbalance survives for even a short time.
Transverse momentum spectra, elliptic flow and interferometry correlation radii
are not very sensitive to this early pressure anisotropy. Comparison with the
data points toward a short equilibration time of the order of $0.25$fm/c or
less.
