Ubuntu 26.04 LTS – system clock drifts because systemd-timesyncd cannot sync

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Administrators observe: Ubuntu 26.04 LTS – system clock drifts because systemd-timesyncd cannot sync. Token validation, TLS, and distributed jobs fail due to skew.

Environment & Reproduction

Issue appears when UDP/123 is blocked, DNS is unstable, or NTP service conflicts exist.

# Baseline context
lsb_release -a
timedatectl status
systemctl status systemd-timesyncd --no-pager
resolvectl query ntp.ubuntu.com || true
sudo journalctl -u systemd-timesyncd -n 120 --no-pager

Root Cause Analysis

Time sync fails when NTP endpoints are unreachable or another daemon controls system time state.

Quick Triage

Check synchronization flag and active NTP service ownership.

# Quick triage
timedatectl show-timesync --all
ss -lunp | grep ':123' || true
systemctl is-active systemd-timesyncd

Step-by-Step Diagnosis

Inspect config and packet path for NTP reachability.

# Detailed diagnosis
cat /etc/systemd/timesyncd.conf
sudo ufw status verbose
resolvectl query time.cloudflare.com
sudo journalctl -b -u systemd-timesyncd --no-pager | tail -n 200

Solution – Primary Fix

Set reliable NTP servers and restart timesyncd.

# Primary fix
sudo sed -i 's/^#NTP=.*/NTP=time.cloudflare.com time.google.com/' /etc/systemd/timesyncd.conf
sudo sed -i 's/^#FallbackNTP=.*/FallbackNTP=ntp.ubuntu.com/' /etc/systemd/timesyncd.conf
sudo systemctl restart systemd-timesyncd
timedatectl set-ntp true

Solution – Alternative Approaches

Use chrony where stricter time discipline is required.

# Alternative
sudo apt-get install -y chrony
sudo systemctl disable --now systemd-timesyncd
sudo systemctl enable --now chrony

Verification & Acceptance Criteria

System reports synchronized clock and low offset over time.

# Verify
timedatectl status
timedatectl show-timesync --all | grep -E 'ServerName|PollIntervalUSec|Frequency'
date -u

Rollback Plan

Revert timesyncd config and service selection if changes conflict with environment policy.

# Rollback
sudo apt-get purge -y chrony || true
sudo systemctl enable --now systemd-timesyncd
sudo systemctl restart systemd-timesyncd

Prevention & Hardening

Monitor offset and enforce NTP reachability in firewall/network policy.

# Hardening
sudo journalctl -u systemd-timesyncd --since '24 hours ago' --no-pager | grep -Ei 'sync|error' || true
ss -lunp | grep ':123' || true

Related Errors & Cross-Refs

Related patterns include Clock unsynchronized, TLS certificate not yet valid, and Kerberos skew errors.

Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.

View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

systemd-timesyncd manual, Ubuntu time synchronization guide, and chrony documentation.