What is timedatectl's definition of "synchronized"?

Question

timedatectl produces the following output:

               Local time: Tue 2022-05-10 01:07:46 UTC
           Universal time: Tue 2022-05-10 01:07:46 UTC
                 RTC time: Tue 2022-05-10 01:07:46    
                Time zone: Etc/UTC (UTC, +0000)       
System clock synchronized: yes                        
              NTP service: active                     
          RTC in local TZ: no

I see that the system clock is synchronized with NTP, but how synchronized is "synchronized"? How frequently is NTP consulted, and if the connection to the NTP server is lost, at what point will the system decide that the clock is no longer in sync?

score 0 · Accepted Answer · answered May 10 '22 at 13:07

timedatectl uses the org.freedesktop.timedate1 D-Bus interface to query the current state. It outputs the results from that, and the specific meaning can vary.

The results typically reflect the status returned by adjtimex. If you’re using timedated, the clock is considered synchronised if adjtimex succeeds and the reported time error is less than 16 seconds. If you’re using timedatex, the clock is considered synchronised if adjtimex succeeds and the reported time error is less than 10 seconds.

How often the time is checked depends on the service you’re using to manage your time (if any).

The kernel has a flag to indicate that time is not synchronised (STA_UNSYNC), but that is also tied to the “11-minute hardware synchronisation” mechanism. It is therefore not uncommon to have systems with synchronised clocks but STA_UNSYNC set (see the comments in the code linked above).

What is timedatectl's definition of "synchronized"?

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