This Recommendation describes how a trusted time source can be provided to a Time Stamp Authority (TSA) and defines the function of a Time Assessment Authority (TAA) which certificates the traceability of the time reference of a TSA to UTC(k) provided by a timing centre.
ITU-R TF.1876-2010 Referenced Document
ITU-R TF.486 Use of UTYC Frequency as Reference in Standard Requency and Time Signals Emissions
ITU-T X.842 Information technology - Security techniques - Guidelines for the use and management of trusted third party services (Study Group 7)