Fedora hardware real-time clock setting for dual-boot with Windows

To maintain consistent wall-clock times between Fedora and Windows, you must ensure both operating systems interpret the hardware real-time clock (RTC) using the same standard. By default, Fedora uses UTC, while Windows uses Local Time.

The Impact of Local RTC on NTP Stability

Executing timedatectl set-local-rtc 1 instructs Fedora to treat the hardware clock as local time. While the system clock continues to be disciplined by NTP (via chronyd) during active sessions, the implications are as follows:

• DST Ambiguity: The RTC has no concept of time zones. During daylight-saving transitions, the hardware clock may be written with a value that is ambiguous or incorrect until the next NTP synchronization occurs.
• Sync Latency: On boot, the system reads the local RTC. If the RTC drifted or was changed by the other OS, there may be a temporary jump in system time once NTP corrects it. This can cause brief inconsistencies in logs or scheduled cron jobs.
• Containerized Workloads: Most containers rely on the host's system clock. As long as NTP is active, containers will see the correct time, but they are susceptible to the same boot-time jumps mentioned above.

Verification Process

To verify that both operating systems are synchronized without manual intervention, follow these steps:

1. In Fedora, apply the local RTC setting:

   timedatectl set-local-rtc 1

2. Verify the setting:

   timedatectl --no-pager

   (Confirm RTC in local TZ: yes).
3. Perform a full shutdown (not a fast-startup hybrid boot in Windows).
4. Boot into Windows, check the clock, and shut down.
5. Boot into Fedora, check the clock using the date command, and shut down.
6. Repeat this cycle across a Daylight Saving Time boundary to ensure neither OS "double-adjusts" the clock.

Recommended Fallback Strategy

If you boot into Windows only occasionally, the most stable approach is to keep Fedora in its native UTC mode and modify the Windows Registry to support UTC. This avoids the DST ambiguities inherent in local RTC mode.

However, if you prefer not to modify the Windows Registry, the timedatectl set-local-rtc 1 command is the standard fallback. It ensures Windows—the more rigid of the two regarding RTC expectations—remains undisturbed.

Diagnostic Detail Required: Are you using Windows "Fast Startup"? If enabled, Windows may not write the final system time to the RTC during shutdown, which can lead to persistent offsets regardless of the timedatectl setting.