Why is the clock off by 8 hours after switching between Windows and Linux on one PC in Taiwan?

Question

Weirdo · Accepted Answer

Because the two systems read the motherboard clock differently. Windows treats it as local time by default, while many Linux systems treat it as UTC; Taiwan is UTC+8, so you see exactly 8 hours of difference after switching.

The ArchWiki explains that the hardware clock (RTC, a.k.a. the BIOS time) is a single clock storing year, month, day, hour, minute, and second; whether it counts as local time or UTC is decided by the operating system. Windows uses local time by default, macOS uses UTC, and other UNIX-like systems vary. For example, if Windows writes Taiwan's 15:00 into the clock and Linux reads it as UTC, Linux adds 8 hours and shows 23:00.

Microsoft's Raymond Chen explains it's mostly a backward-compatibility chain: people used to dual-boot Windows NT with MS-DOS/Windows 3.1, which ran on local time, so Windows NT followed suit, and the choice was preserved through each upgrade so people could keep dual-booting with their previous system. Another reason is avoiding confusion: set the time to 3pm in the BIOS setup screen, boot Windows, and see 11am, and you'd be rightly annoyed.

The ArchWiki's recommendation is to make Windows use UTC: add a DWORD value named RealTimeIsUniversal set to 1 under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation (back up the registry first). Alternatively, run timedatectl set-local-rtc 1 on Linux so it uses local time instead. Pick one or the other. The ArchWiki also notes that some distributions, such as Ubuntu and Fedora, switch to local time when they detect Windows, and that running network time sync on both systems can make them interfere with each other, so keep it on only one.

Why is the clock off by 8 hours after switching between Windows and Linux on one PC in Taiwan?

There's only one clock on the motherboard

Why Windows uses local time

How to fix it