[2] クロックスキュー - Wikipedia
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https://ja.wikipedia.org/wiki/%E3%82%AF%E3%83%AD%E3%83%83%E3%82%AF%E3%82%B9%E3%82%AD%E3%83%A5%E3%83%BC
[3] ntp - Can time skew on Windows be reduced to +/- 5ms? - Server Fault
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http://serverfault.com/questions/147878/can-time-skew-on-windows-be-reduced-to-5ms
[5] Time - ArchWiki
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https://wiki.archlinux.org/index.php/time#Time_skew
Every clock has a value that differs from real time (the best representation of which being International Atomic Time); no clock is perfect. A quartz-based electronic clock keeps imperfect time, but maintains a consistent inaccuracy. This base 'inaccuracy' is known as 'time skew' or 'time drift'.
[6] High Resolution Time Level 2
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https://w3c.github.io/hr-time/#h-introduction
In practice, these definitions of time are subject to both clock skew and adjustment of the system clock. The value of time may not always be monotonically increasing and subsequent values may either decrease or remain the same.
[7] linux - Difference between CLOCK_REALTIME and CLOCK_MONOTONIC? - Stack Overflow
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http://stackoverflow.com/questions/3523442/difference-between-clock-realtime-and-clock-monotonic