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.

Annex A 8.17 of ISO 27001:2022 focuses on clock synchronisation.
At a practical level, this means:
The control does not mandate a specific protocol or technology. It expects consistent, accurate, and defensible time alignment.
Time underpins almost every security activity, including:
If clocks are inconsistent:
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.
A pragmatic approach to Annex A 8.17 typically includes the following elements.
Organisations should define a single, standard reference time that all systems align to.
This reference time should be:
Having a defined reference avoids ambiguity when analysing events.
Annex A 8.17 supports the use of trusted external time sources, such as:
Time sources should be selected based on reliability and resilience, not convenience.
Clock synchronisation should apply to:
Partial synchronisation limits the value of logging and monitoring.
Organisations should use protocols appropriate to their environment, such as:
The protocol choice should reflect:
Precision requirements vary — consistency is the priority.
Where accuracy and availability are critical, organisations should consider:
Time synchronisation should not depend on a single fragile dependency.
Annex A 8.17 expects organisations to ensure synchronisation remains effective.
This may include:
Silent time drift undermines trust in records.
Modern environments often mix:
Organisations should:
Assumed alignment in hybrid environments is a common mistake.
Annex A 8.17 closely supports:
Time consistency is what allows logs from different systems to form a coherent narrative.
Accurate time records may be required for:
Clock synchronisation supports the credibility and admissibility of evidence.
Annex A 8.17 does not require:
It does require organisations to:
When time cannot be trusted, neither can the conclusions drawn from it.
Clock issues are rarely noticed — until an incident occurs.
Annex A 8.17 is about making time a reliable foundation for security.
When clock synchronisation is implemented effectively:
Security depends on evidence.
Evidence depends on time.
Annex A 8.17 ensures organisations get time on their side.
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