ISO 27001:2022 Annex A 8.17 – Clock Synchronisation Explained

When system clocks disagree, the truth becomes hard to prove.

Annex A 8.17 exists to ensure organisations synchronise clocks across information systems, so logs, monitoring, investigations, and evidence all align to a consistent and reliable time reference.

This control is about trust in records, not just technical accuracy.

ISO 27001

Quick Guide: Annex A 8.17 at a Glance

Annex A 8.17 of ISO 27001:2022 focuses on clock synchronisation.

At a practical level, this means:

  • Using a reliable, authoritative time source
  • Synchronising clocks across systems and networks
  • Ensuring logs and events can be correlated accurately
  • Supporting investigations, audits, and legal proceedings
  • Managing time consistency across on-premises and cloud systems

The control does not mandate a specific protocol or technology. It expects consistent, accurate, and defensible time alignment.

In-Depth Guide to Annex A 8.17

What Is Annex A 8.17 and Why Does It Matter?

Time underpins almost every security activity, including:

  • Logging and monitoring
  • Incident detection and response
  • Forensic investigation
  • Legal and regulatory evidence
  • Operational troubleshooting

If clocks are inconsistent:

  • Event timelines become unreliable
  • Logs from different systems cannot be correlated
  • Investigations are delayed or undermined
  • Evidence may be challenged or rejected

Annex A 8.17 ensures organisations can reconstruct events with confidence, even across complex and distributed environments.

This control supersedes ISO 27001:2013 Annex A 12.4.4, with expanded consideration for cloud and hybrid environments.

How to Implement Annex A 8.17 Effectively

A pragmatic approach to Annex A 8.17 typically includes the following elements.

1. Define a Standard Reference Time

Organisations should define a single, standard reference time that all systems align to.

This reference time should be:

  • Reliable
  • Accurate
  • Consistent across the organisation

Having a defined reference avoids ambiguity when analysing events.

2. Use Reliable and Authoritative Time Sources

Annex A 8.17 supports the use of trusted external time sources, such as:

  • National or international time standards
  • Atomic clock–based services
  • Trusted network time services

Time sources should be selected based on reliability and resilience, not convenience.

3. Synchronise Clocks Across All Relevant Systems

Clock synchronisation should apply to:

  • Servers and infrastructure
  • Network devices
  • Security tools and appliances
  • End-user systems where relevant
  • Applications that generate logs or audit records

Partial synchronisation limits the value of logging and monitoring.

4. Apply Appropriate Synchronisation Protocols

Organisations should use protocols appropriate to their environment, such as:

  • Network Time Protocol (NTP)
  • Precision Time Protocol (PTP)

The protocol choice should reflect:

  • Required accuracy
  • Network design
  • System criticality

Precision requirements vary — consistency is the priority.

5. Consider Redundancy of Time Sources

Where accuracy and availability are critical, organisations should consider:

  • Multiple time sources
  • Fallback mechanisms if a primary source fails

Time synchronisation should not depend on a single fragile dependency.

6. Monitor and Validate Clock Synchronisation

Annex A 8.17 expects organisations to ensure synchronisation remains effective.

This may include:

  • Monitoring for drift or loss of synchronisation
  • Alerting when discrepancies exceed defined thresholds
  • Periodic verification checks

Silent time drift undermines trust in records.

7. Address Cloud and Hybrid Environments Explicitly

Modern environments often mix:

  • On-premises systems
  • Cloud-hosted platforms
  • Managed and third-party services

Organisations should:

  • Understand how cloud providers manage time
  • Identify differences between provider and internal time sources
  • Document and manage any unavoidable discrepancies

Assumed alignment in hybrid environments is a common mistake.

8. Align Clock Synchronisation With Logging and Monitoring

Annex A 8.17 closely supports:

Time consistency is what allows logs from different systems to form a coherent narrative.

9. Consider Legal, Regulatory, and Contractual Requirements

Accurate time records may be required for:

  • Regulatory investigations
  • Legal proceedings
  • Contractual dispute resolution

Clock synchronisation supports the credibility and admissibility of evidence.

In-Depth Guide to Annex A 5.1

What is Annex A 5.1 and Why Does It Matter?

  • What the organisation values
  • How security decisions are approached
  • Where accountability sits
  • Aligns security with business objectives
  • Supports proportionate, risk-based decisions
  • Provides a reference point during incidents and disputes
  • Creates consistency without bureaucracy

How to Implement Annex A 5.1 Effectively

1. Define a Comprehensive Information Security Policy
  • Scope: What the policy applies to (systems, data, people).
  • Objectives: Why security is important for the business.
  • Roles & Responsibilities: Who is accountable for security.
  • Key Principles: Risk management, access control, and data protection.
2. Secure Senior Management Approval
  • Review and approve policies.
  • Ensure resources are allocated for implementation.
  • Lead by example in enforcing security policies.
3. Communicate and Train Employees
  • Include security policies in onboarding and ongoing training.
  • Ensure policies are easily accessible to all employees.
  • Encourage acknowledgment and acceptance of security responsibilities.
4. Regular Review & Continuous Improvement
  • Set a schedule for periodic policy reviews (at least annually).
  • Update policies based on business changes, new threats, or regulatory updates.
  • Conduct audits to ensure compliance and effectiveness.
5. Integrate Policies into the ISMS
  • Use it to develop more detailed security procedures.
  • Ensure policies align with other Annex A controls and broader risk management practices.
Key Differences: ISO 27001:2013 vs ISO 27001:2022
AspectISO 27001:2013ISO 27001:2022
Control StructureTwo separate controls: 5.1.1 & 5.1.2Merged into one control (5.1)
Implementation GuidanceLess prescriptiveMore detailed guidance for policy creation and alignment
Awareness & TrainingNot explicitly mentionedExplicitly requires policies to be part of training programmes
Attributes TableNot includedew attributes table for mapping policies to industry terms
Common Challenges & How to Overcome Them
  • Challenge: Employees don’t follow security policies.
  • Solution: Make policies practical and relevant to daily work. Use real-world examples in training.
  • Challenge: Policies are outdated or too generic.
  • Solution: Schedule annual reviews and update policies based on real threats and business changes.
  • Challenge: Policies are written in technical jargon.
  • Solution: Use plain language that all employees can understand.
  • Challenge: Lack of leadership buy-in.
  • Solution: Show how weak security affects business objectives—costs of breaches, loss of client trust, and legal penalties.
Final Recommendations
  • Make security policies living documents—review, refine, and update regularly.
  • Use policy training and awareness to create a security-conscious workforce.
  • Ensure policies are accessible, relevant, and easy to understand.
  • Keep policies aligned with business strategy and regulatory requirements.
  • Engage leadership in driving security culture from the top down.

Practical Considerations

Annex A 8.17 does not require:

  • Ultra-high precision everywhere
  • Specialised hardware by default
  • Manual clock management

It does require organisations to:

  • Avoid inconsistent or drifting clocks
  • Be able to explain how time accuracy is achieved
  • Trust their own logs and records

When time cannot be trusted, neither can the conclusions drawn from it.

Common Challenges and How to Overcome Them

  • Different systems using different time sources
  • Define and enforce a standard reference time
  • No monitoring of time drift
  • Monitor and alert on synchronisation failure
  • Assuming cloud systems are aligned automatically
  • Understand and document provider time handling
  • Inconsistent timestamps across logs
  • Align synchronisation with logging and monitoring controls

Clock issues are rarely noticed — until an incident occurs.

In-Depth Guide to Annex A 5.1

What is Annex A 5.1 and Why Does It Matter?

  • What the organisation values
  • How security decisions are approached
  • Where accountability sits
  • Aligns security with business objectives
  • Supports proportionate, risk-based decisions
  • Provides a reference point during incidents and disputes
  • Creates consistency without bureaucracy

How to Implement Annex A 5.1 Effectively

1. Define a Comprehensive Information Security Policy
  • Scope: What the policy applies to (systems, data, people).
  • Objectives: Why security is important for the business.
  • Roles & Responsibilities: Who is accountable for security.
  • Key Principles: Risk management, access control, and data protection.
2. Secure Senior Management Approval
  • Review and approve policies.
  • Ensure resources are allocated for implementation.
  • Lead by example in enforcing security policies.
3. Communicate and Train Employees
  • Include security policies in onboarding and ongoing training.
  • Ensure policies are easily accessible to all employees.
  • Encourage acknowledgment and acceptance of security responsibilities.
4. Regular Review & Continuous Improvement
  • Set a schedule for periodic policy reviews (at least annually).
  • Update policies based on business changes, new threats, or regulatory updates.
  • Conduct audits to ensure compliance and effectiveness.
5. Integrate Policies into the ISMS
  • Use it to develop more detailed security procedures.
  • Ensure policies align with other Annex A controls and broader risk management practices.
Key Differences: ISO 27001:2013 vs ISO 27001:2022
AspectISO 27001:2013ISO 27001:2022
Control StructureTwo separate controls: 5.1.1 & 5.1.2Merged into one control (5.1)
Implementation GuidanceLess prescriptiveMore detailed guidance for policy creation and alignment
Awareness & TrainingNot explicitly mentionedExplicitly requires policies to be part of training programmes
Attributes TableNot includedew attributes table for mapping policies to industry terms
Common Challenges & How to Overcome Them
  • Challenge: Employees don’t follow security policies.
  • Solution: Make policies practical and relevant to daily work. Use real-world examples in training.
  • Challenge: Policies are outdated or too generic.
  • Solution: Schedule annual reviews and update policies based on real threats and business changes.
  • Challenge: Policies are written in technical jargon.
  • Solution: Use plain language that all employees can understand.
  • Challenge: Lack of leadership buy-in.
  • Solution: Show how weak security affects business objectives—costs of breaches, loss of client trust, and legal penalties.
Final Recommendations
  • Make security policies living documents—review, refine, and update regularly.
  • Use policy training and awareness to create a security-conscious workforce.
  • Ensure policies are accessible, relevant, and easy to understand.
  • Keep policies aligned with business strategy and regulatory requirements.
  • Engage leadership in driving security culture from the top down.

Final Recommendations

Annex A 8.17 is about making time a reliable foundation for security.

When clock synchronisation is implemented effectively:

  • Logs can be trusted
  • Events can be reconstructed accurately
  • Investigations are faster and more credible
  • Legal and regulatory confidence improves

Security depends on evidence.
Evidence depends on time.

Annex A 8.17 ensures organisations get time on their side.

In-Depth Guide to Annex A 5.1

What is Annex A 5.1 and Why Does It Matter?

  • What the organisation values
  • How security decisions are approached
  • Where accountability sits
  • Aligns security with business objectives
  • Supports proportionate, risk-based decisions
  • Provides a reference point during incidents and disputes
  • Creates consistency without bureaucracy

How to Implement Annex A 5.1 Effectively

1. Define a Comprehensive Information Security Policy
  • Scope: What the policy applies to (systems, data, people).
  • Objectives: Why security is important for the business.
  • Roles & Responsibilities: Who is accountable for security.
  • Key Principles: Risk management, access control, and data protection.
2. Secure Senior Management Approval
  • Review and approve policies.
  • Ensure resources are allocated for implementation.
  • Lead by example in enforcing security policies.
3. Communicate and Train Employees
  • Include security policies in onboarding and ongoing training.
  • Ensure policies are easily accessible to all employees.
  • Encourage acknowledgment and acceptance of security responsibilities.
4. Regular Review & Continuous Improvement
  • Set a schedule for periodic policy reviews (at least annually).
  • Update policies based on business changes, new threats, or regulatory updates.
  • Conduct audits to ensure compliance and effectiveness.
5. Integrate Policies into the ISMS
  • Use it to develop more detailed security procedures.
  • Ensure policies align with other Annex A controls and broader risk management practices.
Key Differences: ISO 27001:2013 vs ISO 27001:2022
AspectISO 27001:2013ISO 27001:2022
Control StructureTwo separate controls: 5.1.1 & 5.1.2Merged into one control (5.1)
Implementation GuidanceLess prescriptiveMore detailed guidance for policy creation and alignment
Awareness & TrainingNot explicitly mentionedExplicitly requires policies to be part of training programmes
Attributes TableNot includedew attributes table for mapping policies to industry terms
Common Challenges & How to Overcome Them
  • Challenge: Employees don’t follow security policies.
  • Solution: Make policies practical and relevant to daily work. Use real-world examples in training.
  • Challenge: Policies are outdated or too generic.
  • Solution: Schedule annual reviews and update policies based on real threats and business changes.
  • Challenge: Policies are written in technical jargon.
  • Solution: Use plain language that all employees can understand.
  • Challenge: Lack of leadership buy-in.
  • Solution: Show how weak security affects business objectives—costs of breaches, loss of client trust, and legal penalties.
Final Recommendations
  • Make security policies living documents—review, refine, and update regularly.
  • Use policy training and awareness to create a security-conscious workforce.
  • Ensure policies are accessible, relevant, and easy to understand.
  • Keep policies aligned with business strategy and regulatory requirements.
  • Engage leadership in driving security culture from the top down.

The Annex Control Table

ISO 27001:2022 Organisational Controls
ISO 27001:2022 Technological Controls