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 _ { ens } .
Direct comparison of the terrestial time TT ( BIPM06 ) and the group pulsar time PT _ { 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 _ { 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 } .