17 Clock: Date and Time

20220704

There are two clocks in a computer: the system clock and the hardware clock. The system clock is maintained by the running operating system (e.g., GNU/Linux). It is also said to be maintained by the kernel. The hardware clock is part of the actual hardware of the computer and is usually battery backed up. The current status of the clock can be found from the timedatectl command from systemd:

timedatectl

The output might be something like:

               Local time: Mon 2022-07-04 14:33:47 AEST
           Universal time: Mon 2022-07-04 04:33:47 UTC
                 RTC time: Mon 2022-07-04 04:33:47
                Time zone: Australia/Sydney (AEST, +1000)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

If the system clock is not being synchronised then that can be turned on with:

timedatectl set-ntp yes

To set the timezone:

$ sudo timedatectl set-timezone Australia/Canberra

The date command will also show the current time from the system clock:

date

## Mon Nov 18 03:02:46 PM AEDT 2024

Meanwhile, the hwclock command talks to the hardware clock:

$ sudo hwclock
2021-03-23 07:42:20.426656+11:00

To set the system clock:

$ sudo date -s "13 Jun 2021 10:10:00"

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