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"Agreement" of time is probably nonsense, yeah. I realized after posting so I edited in the parenthetical, but as [3] notes, locality probably makes this less of a real issue.

Apparently with the birthday paradox 32 bit random IDs only allow some tens of thousands per second before collision chance passes 50%. Maybe that's acceptable?

[3]: https://news.ycombinator.com/item?id=47065241

You don't need the universe to agree. You need your ID system to agree within a reasonable margin of error.

The temperature of the cosmic microwave background can be used as a universal clock.

So can neutron star spin rates

Neutron star spins collectively can be used as a pretty accurate clock.

How? You are here on earth, looking at a neutron star. It pulses about N times a minute. I am 5ly away, looking at the same neutron star. Yes, i see the pulses happening at about N times per second, but the pulse delay you see at time T(a) is not the same pulse delay i see at time T(a), it would be another 5 years before i see the same pulse you saw.