Maxim Mironjuk
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March 11, 2026
Alongside the system clock the kernel keeps in memory, almost every physical and virtual machine has a battery backed hardware clock that keeps running even while the machine is powered off. Anyone who does not know the difference between the system clock and the hardware clock will sooner or later be puzzled by wrong boot timestamps, mistimed certificate checks, or a dual boot system that shows a different time after every Windows startup. This article explains how hwclock works, how to set the hardware clock correctly, and how to keep it reliably synchronized with the system clock.