Man-made clocks are more stable than pulsars, thought to be the most accurate timekeepers in the universe.
The best terrestrial artificial clocks substantially exceed the performance of astronomical sources as time-keepers in terms of accuracy and stability.
The best millisecond pulsars have a stability of only one part in 1015.
The best optical lattice neutral atom clocks and trapped ion clocks have a frequency stability approaching one part in 1017.
Earth-bound clocks may be able to outperform pulsars over human time-scales but to do it over significantly longer time-scales is another challenge altogether.