Time in Linux: timedatectl and timezones

timedatectl shows and sets the system time: the timezone and network time sync (NTP).

Common cases

timedatectl                          # time, zone, and whether sync is on
timedatectl list-timezones           # the list of timezones
sudo timedatectl set-timezone Europe/Moscow  # change the zone
sudo timedatectl set-ntp true        # turn on network time sync
date                                 # just the current time

How it works: the system clock and the hardware clock

A computer has two clocks: the hardware clock (it ticks on a battery even with the power off) and the system clock (the kernel keeps it while the system runs). At boot the system clock is set from the hardware one, and after that network time sync keeps it accurate - the NTP service asks time servers on the internet for the time. timedatectl shows both and controls the sync. The timezone is just an offset for display; internally the system usually runs in UTC. So changing the zone is about how time is shown, not about 'moving the clock hands'.

In the book

This topic is covered in full in the chapter Time and synchronization.

See also

The date command.