A clock that changes after booting a second operating system needs a different comparison from an ordinary wrong time zone. Each system has a displayed time, a zone configuration and a convention for interpreting the computer's hardware real-time clock, or RTC.

Use a trusted independent clock as the reference. Record which operating system was running before the change, the size of the difference and whether network synchronisation later corrected it. Avoid manually adjusting both systems in quick succession, because that can hide the sequence you need to understand.

Inspect the Linux side without changing it

Run timedatectl status on the Linux desktop. Its documented status view includes system-clock and RTC settings together with time-synchronisation information. Record the time zone and whether the RTC is treated as local time, along with the observed clock values.

Compare these with the second operating system's documented configuration. Do not assume its convention from its product name or an old setup guide; the computer may already have been customised. A repeatable whole-hour jump after switching systems suggests a convention mismatch worth checking, but it does not prove that cause by itself.

Agree one convention before correcting it

The timedatectl manual defines set-local-rtc 0 as UTC and 1 as local time. It recommends UTC where possible because local-RTC mode has limitations around time-zone and daylight-saving changes. Changing this setting also updates the RTC from the system clock unless a specific alternative adjustment option is used.

That side effect means a copied command is not merely changing a label. Coordinate the intended convention across both operating systems and confirm the correct system time before an authorised correction. On a managed computer, provide the status evidence to the person responsible for its time policy.

After correction, test the original boot sequence in both directions and compare against the reference again. Check the immediate post-boot value as well as the later synchronised value. Preserve any remaining drift or date reset for separate investigation. Success means the convention and repeated switching behaviour agree, not that one desktop briefly shows the right hour.

Sources: Ubuntu documentation.