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So here's the deal -- I'm dual-booting Mac OS X and Windows XP on my MacBook. On Mac OS X, I have the clock synced to network time. And, being a good Unix system, it sets the system time to GMT.

That's all fine and dandy until I install Windows, which uses localtime. I had to disable network time on Windows to keep my time from being screwed up every time I switched operating systems, so at the moment only Windows has its time wrong. I'd like to know of a way to set Windows to use GMT, but from what I've read it isn't possible;

http://blogs.msdn.com/oldnewthing/archive/2004/09/02/224672.aspx

Another reason for keeping the BIOS clock on local time is to avoid confusing people who set their time via the BIOS itself. If you hit the magic key during the power-on self-test, the BIOS will go into its configuration mode, and one of the things you can configure here is the time. Imagine how confusing it would be if you set the time to 3pm, and then when you started Windows, the clock read 11am.

"Stupid computer. Why did it even ask me to change the time if it's going to screw it up and make me change it a second time?"

And if you explain to them, "No, you see, that time was UTC, not local time," the response is likely to be "What kind of totally propeller-headed nonsense is that? You're telling me that when the computer asks me what time it is, I have to tell it what time it is in London? (Except during the summer in the northern hemisphere, when I have to tell it what time it is in Reykjavik!?) Why do I have to remember my time zone and manually subtract four hours? Or is it five during the summer? Or maybe I have to add. Why do I even have to think about this? Stupid Microsoft. My watch says three o'clock. I type three o'clock. End of story."

Has Microsoft in all their wisdom crippled functionality because they thought it wasn't user-friendly? Or am I missing something?

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Dani AI

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Short answer: yes — Windows can be configured to treat the hardware clock as UTC so it agrees with macOS, which avoids the “time flips every reboot” problem that happens when one OS uses local time and the other uses UTC. was on the right track: Windows will honor an explicit flag that tells it to interpret the real-time clock as UTC, which lets both OSes run NTP/network time without fighting each other. (askubuntu.com)

Safe workflow (summary, no raw registry dump here): back up the registry first. Using an elevated regedit, the change is made under the TimeZoneInformation area of the registry by adding a 32-bit value whose name signals “RTC is UTC” and setting it to true; then reboot and re-enable Windows time sync so both OSes can keep accurate time. After the change, confirm the firmware/BIOS clock shows UTC and that both macOS and Windows display the correct local time. Practical examples and editable .reg templates are widely published if a bulk apply is needed. (askubuntu.com)

Important cautions: Microsoft documents real problems when this flag is used on some Windows builds — DST-change bugs, high CPU during DST transitions, and situations where Control Panel time editing is affected — and Microsoft’s KBs/hotfix guidance should be read before rolling this into a production machine. If the machine is domain‑joined or runs time‑sensitive services, test thoroughly; you may need Windows updates or hotfixes for specific builds. If any problems appear, remove the value and reboot to restore default behavior. (support.microsoft.com)

If editing the registry is undesirable, the alternative is to make macOS write the hardware clock as local time (less recommended for Unix-like systems) or keep one OS from auto-syncing and correct the clock manually. For most dual-boot setups with macOS/Linux + Windows, the cleaner, long-term approach is to make both OSes agree on RTC=UTC and let NTP handle drift — but proceed with backups, testing across a DST boundary, and awareness of the Microsoft caveats above. (askubuntu.com)

Member Avatar for John A

Hrm. Maybe I spoke too soon. It seems as though the registry key

HKEY_LOCAL_MACHINE/SYSTEMCurrentControlSetControl/TimeZoneInformation/RealTimeIsUniversal

is what I'm looking for. I'll let you know how it goes.

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