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https://libertysys.com.au/2016/10/the-school-for-sysadmins-who-cant-timesync-good-and-wanna-learn-to-do-other-stuff-good-too-part-2-how-ntp-works/
The School for Sysadmins Who Can't Timesync Good and Wanna Learn To Do Other Stuff Good Too, part 2 - how NTP works