  An algorithm is proposed for constructing a group (ensemble) pulsar time
based on the application of optimal Wiener filters. This algorithm makes it
possible to separate the contributions of variations of the atomic time scale
and of the pulsar rotation to barycentric residual deviations of the pulse
arrival times. The method is applied to observations of the pulsars PSR
B1855+09 and PSR B1937+21, and is used to obtain corrections to UTC relative to
the group pulsar time PT$_{\rm ens}$. Direct comparison of the terrestrial time
TT(BIPM06) and the group pulsar time PT$_{\rm ens}$ shows that they disagree by
no more than $0.4\pm 0.17\; \mu$s. Based on the fractional instability of the
time difference TT(BIPM06) -- PT$_{\rm ens}$, a new limit for the energy
density of the gravitational-wave background is established at the level
$\Omega_g {h}^2\sim 10^{-9}$.
