The international time and frequency community is progressing towards a future redefinition of the SI second based on optical frequency standards. This development represents one of the most significant changes to the International System of Units since the recent redefinition of the kilogram and has important implications for international timekeeping, metrological traceability and the realization of Coordinated Universal Time (UTC).

This Special Session will bring together national metrology institutes, international organizations, universities, industry and infrastructure operators to discuss the scientific foundations, metrological requirements and practical implementation challenges associated with a future redefinition. Topics will include historical studies on the previous redefinitions of the second, optical clocks, frequency-ratio measurements, validation of candidate standards, international comparisons, timescale generation, UTC realization and dissemination, advanced time-transfer techniques and implementation strategies for users of timing services.

Research Topics: historical aspects, optical frequency standards, roadmap towards the redefinition of the second, realization of the future SI second, UTC following redefinition, time and frequency transfer, applications and societal impact, capacity building and international cooperation.