Keep the system time right

$ run as a normal user# run as root (or with sudo)

Exam objectives in this chapter

Set up time service clients

Understand local time

A computer has two clocks. The hardware clock (RTC) runs on the mainboard battery also when the machine is off. The system clock is kept by the kernel while Linux runs; it starts from the hardware clock at boot.

Clock	Where	Tool
Hardware clock (RTC)	Mainboard, battery	`hwclock`
System clock	Kernel	`date`, `timedatectl`

Linux keeps the system clock in UTC and shows it in your time zone.
The hardware clock should also be UTC (RTC in local TZ: no).

25.2

Use network time

NTP (Network Time Protocol) syncs the clock with time servers. RHEL uses chrony (chronyd) as NTP client and server. Servers are ordered in strata: stratum 1 has a real reference clock (GPS, atomic); stratum 2 syncs from stratum 1, and so on.

$systemctl status chronyd
$chronyc sources -v
$chronyc tracking

25.3

Show and set the time with date

date shows the system time in any format, and root can set it (better: let NTP do it).

$date
Sun Oct 25 11:42:10 CET 2026
$date +%F_%H%M
2026-10-25_1142
$date -u
$date -d '+3 days'
$date -d @1761389000
#date -s "2026-10-25 12:00:00"

Format	Gives
`%F`	2026-10-25
`%T`	11:42:10
`%s`	Seconds since 1970 (epoch)
`%A`, `%B`	Day and month name

25.4

Use hwclock

hwclock reads and sets the hardware clock, and copies time between the two clocks.

#hwclock
#hwclock --systohc
#hwclock --hctosys

Note

chronyd keeps the hardware clock in sync by itself when rtcsync is in /etc/chrony.conf (RHEL default).

25.5

Use timedatectl

timedatectl shows all time settings at once and changes time, time zone and NTP use.

$timedatectl
               Local time: Sun 2026-10-25 11:42:10 CET
           Universal time: Sun 2026-10-25 10:42:10 UTC
                 RTC time: Sun 2026-10-25 10:42:10
                Time zone: Europe/Amsterdam (CET, +0100)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no
#timedatectl set-ntp true
#timedatectl set-ntp false
#timedatectl set-time "2026-10-25 12:00:00"

Watch out

set-time only works when NTP is off. With NTP on, chrony corrects the clock anyway.

25.6

Manage the time zone

The time zone is the link /etc/localtime → a file in /usr/share/zoneinfo/. Change it with timedatectl set-timezone.

$timedatectl list-timezones | grep -i riyadh
Asia/Riyadh
#timedatectl set-timezone Asia/Riyadh
$ls -l /etc/localtime
$TZ=America/New_York date
$tzselect

Note

tzselect only helps you find the name (with questions); it does not change the system.

25.7

Configure time service clients

Tell chrony which time servers to use in /etc/chrony.conf, restart it, and check the sources.

Set the server (replace the pool line or add a server line).

/etc/chrony.conf (part)

# pool 2.rhel.pool.ntp.org iburst
server 192.168.122.1 iburst
driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync

Restart chrony and open the firewall only if this host is a time server for others.
```
#systemctl restart chronyd
#chronyc sources -v
#chronyc tracking
```
Make the clock step at once if it is far off.
```
#chronyc makestep
```

chronyc output	Meaning
`^*`	The source chrony uses now
`^+`	A good source that is combined
`^?` / `^x`	Not reachable / falseticker (wrong time)
`Leap status: Normal` (tracking)	Synchronized

Exam tip

"Configure the system as an NTP client of classroom.example.com" = a server classroom.example.com iburst line in /etc/chrony.conf, systemctl restart chronyd, then check chronyc sources shows ^*.

At work

To make a server an NTP server for your network, add allow 192.168.122.0/24 to chrony.conf and open the firewall service ntp.

25.8

Remember the main points

The most important facts of this chapter, in short.

A system has two clocks: the hardware clock (RTC, on the mainboard) and the system clock (kernel).
Linux keeps the system clock in UTC and shows it in the local time zone.
timedatectl shows everything; timedatectl set-timezone Area/City changes the zone; timedatectl set-ntp true turns on time sync.
date shows and formats the time; hwclock reads and sets the hardware clock.
RHEL syncs time with chrony (chronyd); servers are set in /etc/chrony.conf with server NAME iburst or pool NAME iburst.
After a change: systemctl restart chronyd, then check with chronyc sources -v (^* = the source in use) and chronyc tracking.

25.9

Learn the key terms

The words of this chapter in simple English.

Term	Meaning
Hardware clock (RTC)	The clock on the mainboard that runs when the computer is off.
System clock	The clock of the running kernel.
UTC	Coordinated Universal Time: the base time without time zones.
Time zone	The local time rule of a place, like Asia/Riyadh.
NTP	Network Time Protocol: sync the clock over the network.
chronyd	The NTP service of RHEL.
Stratum	How far a time server is from a real reference clock.
iburst	Send the first requests fast to sync quickly.