NTP — Time Synchronization

Why is time synchronization required?

• TLS/SSL certificates depend on accurate time
• Authentication systems like Kerberos & LDAP require synchronized clocks
• System logs, application logs, and audit logs rely on timestamps
• File Systems & Build Systems generate timestamps
• Distributed Systems (HPC, Kubernetes) & Databases require consistent time

Most modern Linux systems use one of the following components to sync time:

• systemd-timesyncd
• chrony (chronyd) – recommended for servers
• ntpd (older systems)

Check current time status:

>>> timedatectl status
               Local time: Thu 2026-02-12 09:23:41 CET
           Universal time: Thu 2026-02-12 08:23:41 UTC
                 RTC time: Thu 2026-02-12 08:23:41
                Time zone: Europe/Berlin (CET, +0100)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

Types of clocks:

• RTC (Real-Time Clock) aka hardware clock runs on battery when the machine is powered off (unplugged)
  • Uses local time (in current time zone)
  • Or UTC (Coordinated Universal Time) with an time zone dependent offset
• SC (Software Clock) aka system clock
  • Maintained by the OS kernel during run-time driven by a timer interrupt
  • Initialized on boot using RTC as reference.

Chrony

Command	Description
chronyd	The daemon (runs in background)
chronyc	Command-line client to control/query it

Show synchronization status (stratum, drift, current offset

>>> chronyc tracking
Reference ID    : 8CB55A16 (ntp.example.org)
Stratum         : 3
Ref time (UTC)  : Thu Feb 12 08:28:06 2026
System time     : 0.000000040 seconds slow of NTP time
Last offset     : +0.000039854 seconds
RMS offset      : 0.000029645 seconds
Frequency       : 60.475 ppm slow
Residual freq   : +0.033 ppm
Skew            : 0.109 ppm
Root delay      : 0.016435383 seconds
Root dispersion : 0.023702158 seconds
Update interval : 130.6 seconds
Leap status     : Normal

• Leap status — Normal means system is synced
• Stratum — Level in the NTP hierarchy
  • 0 highly accurate reference clocks (atomic & GPS clocks)
  • 1 NTP servers directly connected to stratum 0 devices
  • 2 NTP servers sync their time from stratum 1 servers
  • 3+ NTP servers sync their time from stratum 2 servers, and so on
• Offset — Time difference between client and reference NTP server

Configuration

Main configuration file for Chrony:

/etc/chrony.conf

server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

driftfile /var/lib/chrony/drift
makestep 1.0 3
rtcsync

Restart chronyd so the changes in the config file take effect:

• server — Specify a remote NTP server (hostname or IP address)
  • iburst First initial synchronization faster
  • Multiple servers to increase redundancy and improve accuracy
• driftfile — Information about how much your system’s clock is drifting
• makestep — How to to make adjustments to the system clock
• rtcsync — System time is written to the hardware clock periodically

Verify NTP sources:

chronyc sources -v

systemd-timesyncd

timedatectl                                 # show time and time zone configuration
timedatectl set-time YYYY-MM-DD             # change the current date
timedatectl set-time HH:MM:SS               # change the current time
timedatectl list-timezones                  # list available time zones
timedatectl set-timezone <zone>             # set a given time zone, e.g. Europe/Berlin
grep ^Servers /etc/systemd/timesyncd.conf   # list time servers 
timedatectl set-ntp true                    # enable NTP
timedatectl set-local-rtc 0                 # RTC in UTC mode
timedatectl set-local-rtc 1                 # RTC in locel time mode
systemctl start systemd-timesyncd           # start the time sync daemon 
systemctl enable systemd-timesyncd          # make the time sync daemon boot persistant