Of the various internet .+P, NTP is one I never learned about as a student, so now I'm looking at its web page [1] by its creator David L. Mills (1938-2024). I've found one video of him giving a retrospective of his extensive internet work; he talks about NTP at 34:51 [2] and later at 56:26 [3].
[1] https://www.eecis.udel.edu/~mills/ntp.html
In [3] he mentions that one can use NTP to observe frequency deviations and use it as an early warning system for fire and AC failure. That really intrigues me. Can you actually? Has this ever been implemented?
HN discussion shortly after Dave Mills died, early in 2024: https://news.ycombinator.com/item?id=39051246