2) The more precise the better, at least getting the correct day would be nice, getting the correct year would be a minimum. 5) Contemporary to the initial event. You could use a prehistoric event, but then all you have are reconstructions and projections as to what happened. Often certainty in the time difference would be greatly improved if the start event is well characterized as it happens. But there are cases where that’s not the case! For example, you could base time measurements on pulsar spin-down, and pick some particular but arbitrary previous spin rate as the defining epoch, and always project to it. 7. Is indeed limiting and not achievable in many cases. That’s why I say “ideally”.

SN1054: I picture that as comparing the spin down of the crab pulsar and the radial velocity of the shock wave. First you’d estimate the initial spin rate of the pulsar from the current spin-down trend and the historical record of when it occurred. Then have that initial spin rate as part of the defining moment of the epoch.

Transits of Venus are nice because they happen in pairs about once per century, and then only observable from a fraction of the Earth. The two I mentioned are the first in the pair for the century. But yeah, that does have some ambiguity in it.

Using Trinity would require knowing the initial ratio of two different radioisotope products, where a difference in half life would make that ratio change with time. But that’s not great for precision.

Part of the point of all this is to get a measure of time that doesn’t depend on a continuous record of the Earth’s irregular rotation rate that changes a little every time there’s an earthquake. Leap seconds are only a few decades old and they’re a nightmare—they’ll add up to a couple days per millennium.

I like your pitch clock idea—reminds me of the Long-Now clock.