  Timing of highly stable millisecond pulsars provides the possibility of
independently verifying terrestrial time scales on intervals longer than a
year. An ensemble pulsar time scale is constructed based on pulsar timing data
obtained on the 64-m Parkes telescope (Australia) in 1995--2010. Optimal Wiener
filters were applied to enhance the accuracy of the ensemble time scale. The
run of the time--scale difference PT$_{ens}$ $-$ TT(BIPM2011) does not exceed
$0.8 {\pm} 0.4$ $\mu s$ over the entire studied time interval. The fractional
instability of the difference $_{ens}$ $-$ TT(BIPM2011) over 15 years is
${\sigma_z} = (0.6\pm 1.6){\cdot}10^{-15}$, which corresponds to an upper limit
for the energy density of the gravitational--wave background ${\Omega_g}h^2
{\sim} 10^{-10}$ and variations in the gravitational potential ${\sim}
10^{-15}$ Hz at the frequency $2{\cdot}10^{-9}$.
