Set and Synchronize System Time Using Time Servers¶

Overview¶

This guide covers time synchronization, NTP (Network Time Protocol), Chrony, and time zone management in Linux systems.

Time Concepts¶

System Time: Current time maintained by the kernel
Hardware Clock (RTC): Real-Time Clock, battery-powered clock in the hardware
UTC: Coordinated Universal Time (standard reference)
Local Time: Time adjusted for timezone
NTP: Network Time Protocol for time synchronization
Stratum: Distance from reference clock (lower is better, 0 is atomic clock)

View Current Time¶

`date` Command¶

# Show current date and time
date

# Show in UTC
date -u

# Show in specific format
date "+%Y-%m-%d %H:%M:%S"
date "+%A, %B %d, %Y"

# Show time in seconds since epoch (1970-01-01)
date +%s

# Show date from timestamp
date -d @1609459200

`timedatectl` Command (systemd)¶

# Show time and date settings
timedatectl

# Output includes:
# - Local time
# - Universal time (UTC)
# - RTC time
# - Time zone
# - NTP enabled status
# - NTP synchronized status

# Show available timezones
timedatectl list-timezones

# Show time status in detail
timedatectl status

Set System Time¶

Manual Time Setting¶

Using `date`¶

# Set time manually (format: MMDDhhmmYYYY.ss)
date 123123592024.00  # December 31, 23:59:00, 2024

# Set from string
date -s "2024-12-31 23:59:00"
date -s "31 DEC 2024 23:59:00"

# Set date only
date --set="2024-12-31"

# Set time only
date --set="23:59:00"

Using `timedatectl`¶

# Set date and time
timedatectl set-time "2024-12-31 23:59:00"

# Set date only
timedatectl set-time "2024-12-31"

# Note: NTP must be disabled to set time manually
timedatectl set-ntp false

Time Zone Management¶

View Time Zone¶

# Using timedatectl
timedatectl

# Check timezone file
ls -l /etc/localtime

# View timezone name
cat /etc/timezone  # Debian/Ubuntu

Set Time Zone¶

Using `timedatectl`¶

# List all available timezones
timedatectl list-timezones

# Filter timezones
timedatectl list-timezones | grep America
timedatectl list-timezones | grep Europe/

# Set timezone
timedatectl set-timezone America/New_York
timedatectl set-timezone Europe/London
timedatectl set-timezone Asia/Tokyo
timedatectl set-timezone UTC

# Verify
timedatectl

Manual Method¶

# Create symbolic link to timezone file
ln -sf /usr/share/zoneinfo/America/New_York /etc/localtime

# Update /etc/timezone (Debian/Ubuntu)
echo "America/New_York" > /etc/timezone

Hardware Clock (RTC)¶

`hwclock` Command¶

# Show hardware clock time
hwclock --show
hwclock -r

# Show in UTC
hwclock --show --utc

# Show in local time
hwclock --show --localtime

# Set hardware clock from system time
hwclock --systohc

# Set system time from hardware clock
hwclock --hctosys

# Set hardware clock manually
hwclock --set --date="2024-12-31 23:59:00"

Hardware Clock Mode¶

# Check if RTC is in UTC or local time
timedatectl | grep "RTC in local TZ"

# Set RTC to UTC (recommended)
timedatectl set-local-rtc 0

# Set RTC to local time (not recommended, can cause issues)
timedatectl set-local-rtc 1

NTP - Network Time Protocol¶

Check NTP Status¶

# Using timedatectl
timedatectl status
timedatectl timesync-status

# Show NTP service status
timedatectl show-timesync --all

Enable/Disable NTP¶

# Enable NTP synchronization
timedatectl set-ntp true

# Disable NTP synchronization
timedatectl set-ntp false

# Check if enabled
timedatectl | grep "NTP service"

Chrony (Modern NTP Implementation)¶

Chrony Overview¶

Chrony is the modern replacement for ntpd, designed for: - Better performance on systems that are intermittently connected - Faster synchronization - Better handling of network congestion - Works well with virtual machines

Installation¶

# RHEL/CentOS/Fedora
dnf install chrony

# Ubuntu/Debian
apt install chrony

Chrony Service Management¶

# Start chronyd service
systemctl start chronyd

# Stop chronyd service
systemctl stop chronyd

# Restart chronyd service
systemctl restart chronyd

# Enable at boot
systemctl enable chronyd

# Check status
systemctl status chronyd

# Check if running
systemctl is-active chronyd

Chrony Configuration File¶

Location: /etc/chrony.conf (RHEL) or /etc/chrony/chrony.conf (Ubuntu)

Example configuration:

# Use public NTP servers from pool.ntp.org
pool pool.ntp.org iburst
pool 0.pool.ntp.org iburst
pool 1.pool.ntp.org iburst

# Or use specific servers
server 0.rhel.pool.ntp.org iburst
server 1.rhel.pool.ntp.org iburst
server time.google.com iburst

# Record rate of clock gain/loss
driftfile /var/lib/chrony/drift

# Allow system clock to be stepped in the first three updates
makestep 1.0 3

# Enable RTC synchronization
rtcsync

# Serve time to local network (optional)
allow 192.168.1.0/24

# Log directory
logdir /var/log/chrony

Configuration options explained: - pool: Use pool of NTP servers - server: Use specific NTP server - iburst: Send burst of packets at startup for faster sync - makestep: Allow clock to be stepped if offset is large - driftfile: File to record clock drift rate - rtcsync: Enable kernel RTC synchronization - allow: Allow clients from this network

`chronyc` Command (Chrony Client)¶

View Time Sources¶

# Show NTP sources
chronyc sources

# Show sources with verbose output
chronyc sources -v

# Columns explained:
# M: Mode (^ = server, = = peer)
# S: State (* = current sync source, + = acceptable, - = not acceptable)
# Stratum: Distance from reference clock
# Reach: Reachability (octal, 377 = all 8 recent attempts successful)
# LastRx: Time since last sample
# Poll: Polling interval

# Show detailed source information
chronyc sourcestats

# Show source information with verbose output
chronyc sourcestats -v

Tracking Information¶

# Show system clock performance
chronyc tracking

# Output includes:
# - Reference ID and stratum
# - System time offset
# - Root delay and dispersion
# - Update interval
# - Leap status

Manual Time Synchronization¶

# Force immediate synchronization
chronyc makestep

# Add new server temporarily
chronyc add server time.google.com

# Delete server
chronyc delete time.google.com

Activity and Statistics¶

# Show how many servers/peers are online
chronyc activity

# Show NTP authentication status
chronyc authdata

# Show client access
chronyc clients

Testing and Diagnostics¶

# Perform burst of measurements
chronyc burst 3/5

# Check server reachability
chronyc reachability

# Dump all measurements
chronyc ntpdata

Legacy NTP Daemon (ntpd)¶

Installation¶

# RHEL/CentOS/Fedora
dnf install ntp

# Ubuntu/Debian
apt install ntp

Configuration¶

Location: /etc/ntp.conf

Example configuration:

# Use public servers
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst

# Drift file
driftfile /var/lib/ntp/drift

# Access restrictions
restrict default nomodify notrap nopeer noquery
restrict 127.0.0.1
restrict ::1

# Allow local network to query
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

Service Management¶

# Start service
systemctl start ntpd

# Enable at boot
systemctl enable ntpd

# Check status
systemctl status ntpd

`ntpq` Command¶

# Show peers
ntpq -p

# Show more detailed peer information
ntpq -pn

# Interactive mode
ntpq
ntpq> peers
ntpq> associations
ntpq> quit

`ntpdate` Command (deprecated)¶

# Synchronize time once (deprecated, use chronyc or ntpd)
ntpdate pool.ntp.org

# Note: Cannot run while ntpd is running
systemctl stop ntpd
ntpdate pool.ntp.org
systemctl start ntpd

Systemd-timesyncd (Lightweight NTP)¶

Overview¶

Simple SNTP client included with systemd, used when neither Chrony nor ntpd is installed.

Configuration¶

Location: /etc/systemd/timesyncd.conf

[Time]
NTP=0.pool.ntp.org 1.pool.ntp.org
FallbackNTP=time.google.com

Service Management¶

# Start service
systemctl start systemd-timesyncd

# Enable at boot
systemctl enable systemd-timesyncd

# Check status
systemctl status systemd-timesyncd

# View sync status
timedatectl timesync-status

Public NTP Server Pools¶

Common NTP Servers¶

# Generic pool
pool.ntp.org
0.pool.ntp.org
1.pool.ntp.org
2.pool.ntp.org
3.pool.ntp.org

# Regional pools
us.pool.ntp.org
europe.pool.ntp.org
asia.pool.ntp.org

# Vendor-specific pools
0.rhel.pool.ntp.org
0.ubuntu.pool.ntp.org
0.centos.pool.ntp.org

# Public servers
time.google.com
time.cloudflare.com
time.nist.gov

Firewall Configuration for NTP¶

# Allow NTP traffic (UDP port 123)
firewall-cmd --permanent --add-service=ntp
firewall-cmd --reload

# Or manually with port
firewall-cmd --permanent --add-port=123/udp
firewall-cmd --reload

# For iptables
iptables -A INPUT -p udp --dport 123 -j ACCEPT
iptables -A OUTPUT -p udp --sport 123 -j ACCEPT

Troubleshooting Time Synchronization¶

Check Time Status¶

# Overall status
timedatectl status

# Chrony tracking
chronyc tracking
chronyc sources -v

# Check service
systemctl status chronyd
journalctl -u chronyd -f

Common Issues and Solutions¶

1. Time Not Synchronizing¶

# Check if NTP is enabled
timedatectl | grep "NTP service"

# Enable if disabled
timedatectl set-ntp true

# Restart chronyd
systemctl restart chronyd

# Check sources
chronyc sources -v

# Force synchronization
chronyc makestep

2. Large Time Offset¶

# Check current offset
chronyc tracking

# If offset is very large, may need to step
chronyc makestep

# Or restart service
systemctl restart chronyd

3. No Sources Available¶

# Check network connectivity
ping pool.ntp.org

# Check DNS resolution
nslookup pool.ntp.org

# Check firewall
firewall-cmd --list-all | grep ntp

# Test NTP port
nc -u pool.ntp.org 123

4. Sources Not Reachable¶

# Check sources
chronyc sources

# Look at Reach column (should be 377 for good connectivity)
# If showing 0, check:
# - Firewall rules
# - Network connectivity
# - Server configuration

# Try different servers
chronyc add server time.google.com

Log Files¶

# Chrony logs
journalctl -u chronyd
tail -f /var/log/chrony/*.log

# System logs
journalctl -xe

# Check for time-related errors
journalctl | grep -i "time\|clock\|ntp\|chrony"

Best Practices¶

Use Chrony instead of ntpd for most modern systems
Use pool servers (pool.ntp.org) for redundancy
Use iburst option for faster initial synchronization
Set RTC to UTC to avoid timezone issues
Monitor synchronization regularly with chronyc tracking
Allow firewall access for NTP (UDP 123)
Use regional pools for better latency
Keep chronyd running continuously for best accuracy
Check logs regularly for synchronization issues
Enable at boot to ensure time is correct after restart

Quick Reference Commands¶

View Time¶

date                          # Current date/time
timedatectl                   # Full time status
timedatectl status            # Detailed status
hwclock --show                # Hardware clock

Set Time¶

timedatectl set-time "YYYY-MM-DD HH:MM:SS"
timedatectl set-timezone America/New_York
timedatectl set-ntp true

Chrony¶

systemctl start chronyd       # Start service
chronyc sources               # Show sources
chronyc tracking              # Show sync status
chronyc makestep              # Force sync

Troubleshooting¶

timedatectl timesync-status   # Sync status
journalctl -u chronyd         # View logs
chronyc sources -v            # Detailed sources
chronyc activity              # Source activity

Exam Tips¶

Know how to use both timedatectl and chronyc
Understand the difference between system time and hardware clock
Be able to configure timezone correctly
Know how to enable/disable NTP synchronization
Understand chrony.conf configuration options
Be able to troubleshoot synchronization issues
Remember the iburst option for faster sync
Know how to check synchronization status
Understand the importance of UTC for RTC
Practice manual time setting (with NTP disabled)

Set and Synchronize System Time Using Time Servers¶

Overview¶

Time Concepts¶

View Current Time¶

date Command¶

timedatectl Command (systemd)¶

Set System Time¶

Manual Time Setting¶

Using date¶

Using timedatectl¶

Time Zone Management¶

View Time Zone¶

Set Time Zone¶

Using timedatectl¶

Manual Method¶

Hardware Clock (RTC)¶

hwclock Command¶

Hardware Clock Mode¶

NTP - Network Time Protocol¶

Check NTP Status¶

Enable/Disable NTP¶

Chrony (Modern NTP Implementation)¶

Chrony Overview¶

Installation¶

Chrony Service Management¶

Chrony Configuration File¶

chronyc Command (Chrony Client)¶

View Time Sources¶

Tracking Information¶

Manual Time Synchronization¶

Activity and Statistics¶

Testing and Diagnostics¶

Legacy NTP Daemon (ntpd)¶

Installation¶

Configuration¶

Service Management¶

ntpq Command¶

ntpdate Command (deprecated)¶

Systemd-timesyncd (Lightweight NTP)¶

Overview¶

Configuration¶

Service Management¶

Public NTP Server Pools¶

Common NTP Servers¶

Firewall Configuration for NTP¶

Troubleshooting Time Synchronization¶

Check Time Status¶

Common Issues and Solutions¶

1. Time Not Synchronizing¶

2. Large Time Offset¶

3. No Sources Available¶

4. Sources Not Reachable¶

Log Files¶

Best Practices¶

Quick Reference Commands¶

View Time¶

Set Time¶

Chrony¶

Troubleshooting¶

Exam Tips¶

`date` Command¶

`timedatectl` Command (systemd)¶

Using `date`¶

Using `timedatectl`¶

Using `timedatectl`¶

`hwclock` Command¶

`chronyc` Command (Chrony Client)¶

`ntpq` Command¶

`ntpdate` Command (deprecated)¶