Hi @johngbell95 yep, millisecond pulsars (which are billions of years old and have stabilised over that time) have been shown to be as or better than terrestrial atomic clocks over the long term.

It's rare for them to glitch (like the young pulsars) and their spin down rates are much better quantified.

When they become millisecond pulsars, they have finished their accretion phase (so, we wouldn't use accretion powered pulsars in this, only rotation powered pulsars).