Ask HN: Is it possible for the world to lose track of what time it is?
Say there's some world wide computer glitch and the clocks have to be reset. Is it possible that we lose track of exactly how many seconds passed since the epoch?
The general kind of instrument you want is the https://en.wikipedia.org/wiki/Transit_instrument -- if you can measure the transit of a star to 1 arcsecond, that's better than 1 second/day. That article says there are transit instruments that work down to .01 arcsecond.
Unless you have to go a very long time just counting days (enough for a half a leap second to accumulate), it seems pretty clear you could get the correct UTC second.
You mention computer glitches; there also exist non-transistorized clocks that are accurate to much more than 1s/day. In fact, the Wikipedia article about the Elgin Observatory talks about how use of a transit instrument to discipline a mechanical clock was accurate to .01s https://en.wikipedia.org/wiki/Elgin_National_Watch_Company_O... -- though I doubt such a system is in continuous use today.
Just checked out https://www.nist.gov/pml/time-and-frequency-division/time-re...
I didn't understand everything, but it looks like they have a fleet of atomic clocks for UTC and are resilient to one of the clocks failing. I guess it is possible that if all of those clocks go down, the world loses track of time. I wouldn't want to be responsible for that cluster :)
If you have two databases running on two different computers, there's no guarantee their timestamps are in sync relative to each other or relative to a particular atomic clock's epoch. Is it within a second? Probably, on a modern operating system. Same millisecond? Very unlikely.