What is RTCTimeUSec shown by `timedatectl show`?

On assorted flavors and versions of Linux, I see the following when issuing the timedatectl show command:

Timezone=America/Los_Angeles
LocalRTC=yes
CanNTP=yes
NTP=yes
NTPSynchronized=yes
TimeUSec=Mon 2021-11-22 08:33:06 PST
RTCTimeUSec=Mon 2021-11-22 00:33:06 PS

This value appears to be the local time offset by -8 hrs (my current GMT offset), which doesn't make sense to me.

I can't find any reference to this output in Google or man timedatectl. Could be an artifact of set-local-rtc=1 (dual booting with Windows)?

1 Answer

From

Timezone shows the currently configured time zone. LocalRTC shows whether the RTC is configured to use UTC (false), or the local time zone (true). CanNTP shows whether a service to perform time synchronization over the network is available, and NTP shows whether such a service is enabled.

NTPSynchronized shows whether the kernel reports the time as synchronized (c.f. adjtimex(3)). TimeUSec and RTCTimeUSec show the current time on the system and in the RTC. The purpose of those three properties is to allow remote clients to access this information over D-Bus. Local clients can access the information directly.

If you have Windows still set to local time, this is definitely the cause of the two times differing.

With LocalRTC=no, TimeUSec and RTCTimeUSec have the same value.

To keep the time of the RTC at Universal time for both systems in the dual boot, it's better to force Windows to use UTC as well.

You can do this with the registry value 1 in RealTimeIsUniversal in the Windows registry at HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation.