That said, how would this work for someone on a planet very close to a black hole's event horizon say? Would they still be able to use it?

To be clear, I am deeply ignorant of the math and only have a vague understanding of the physics.

@terdon Alex got the tone that he(!) set for the conversation. Give, and ye shall get what ye gave.

@terdon As for that — literal — edge case... I have no idea, and I do not really care, because the amount of habitable planets sitting on black hole event horizons I expect to be very small. And getting out of that gravity well to sync your local clock against an unaffected time source that uses the Pulsar scheme, ought to suffice.

@MichaelK Well if you want a universal clock, that's the kind of thing you need to deal with, not black holes specifically sure, but different frames of reference. If I experience 3 seconds between "ticks" and you experience three minutes, how do we sync? That's the bit I don't get in your answer.

@terdon In the same manner we do for GPS satellites. They operate in a frame of reference that is different enough from the ground-based frame of reference to lead to clock drift. If you know what the frame of reference is in relation to another, adjusting for the difference is trivial.

@MichaelK My research in undergrad was on pulsars, they are no where near as accurate or universal over long stretches of time as you think. After only 10,000 years roughly we should expect a majority of pulsars to have an unpredictable period. The Voyager probes were made in the 60s mind you, when we had very little understanding of pulsars in general.

@MichaelK We normalize those with respect to the Earth. How would you normalize in the absence of a common frame of reference? The whole issue here is that you don't have one and need to get one, so how can you then normalize?

I was thinking you could instead find a political solution: planet X is the dominant cultural/military/political force and their time was arbitrarily chosen as the Universal Time. That still wouldn't let you have any useful clocks, but it could at least give you a universal time stamp.

@AnthonyKhodanian To be fair, a clock that works well for 10000 years is likely good enough. If pulsars could work in the timescale of a few decades, that should be fine.

@terdon If we are assuming no FTL travel or communication, then no a clock accurate to 10,000 years is not good enough. Someone on the other side of the galaxy from you say 50,000 LY away, sees the Pulsar right next to you as it was 50,000 years ago, and the rate that it had then would be completely different than the one you measure now. If it were a linear process you could calculate the difference, but we know now that pulsar quakes are common and chaotic, and ruin pulsar timings over time

@AnthonyKhodanian Sigh, of course. I was assuming a common frame of reference while trying to work out a common frame of reference. Yes indeed, at the distances we're talking about, 10k is nothing. Fair enough.

@terdon You tell me "You done wrong", and when then I ask for an explanation so I can avoid doing it again , you just delete things and, now I have no idea what it is I have done wrong. Thanks...

Also, your objections... I say the same as I did to Alex: you arguing "Well I cannot see how it can be done", is not an argument for anything. All it does, is demonstrate a negative, down-putting attitude, fuelled by what I can kindly assume is lack of imagination or knowledge; less kindly as a deliberate refusal to see solutions for reasons best left unspeculated lest I get mod-slapped again.

You nearly gave the answer to the "problem" yourself: the answer yourself: we selected a common reference: the Earth. So, we can select a common reference in the galaxy and make it part of the time-keeping standard. No-where in the question was is said that every civilisation must work out a common time reference without consensus or communication with the others.

In any case... you got what you wanted: answer removed. Go celebrate. Bye.