Terrestrial timescales show instabilities due to the physical limitations of the atomic clocks. Stricter environmental isolation and increased numbers of improved cesium clocks and cavity-tuned hydrogen masers have resulted in time scales more accurate by a factor of about ve. The use of dierent clock ensembles results in measurable changes in some millisecond pulsar timing data. We i n v estigate the possible application of millisecond pulsars to dene a precise long-term time standard and

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... ional reference system in a nearly inertial reference frame. Although possible quantitative contribution of the two longest studied millisecond pulsars to terrestrial timescales appears minimal, they may prove useful as independent standards in identifying error sources that are dicult to detect due to the nite lifetime and common reference frame of terrestrial clocks. New millisecond pulsars, perhaps some with even better timing properties, may be discovered as a result of the current global pulsar search eorts.