Optical atomic clocks have now reached levels of performance that clearly surpass the stability and accuracy achievable with caesium microwave primary frequency standards, with the result that a future optical redefinition of the SI second is anticipated. This three-year collaborative project will tackle key steps that must be taken prior to the redefinition. The robustness of optical clocks will be improved so that they can run unattended for long periods, methods will be developed for automated data validation, and the long-term reliability and reproducibility of the clocks will be assessed through a coordinated programme of international comparisons. By the end of the project, several European optical clocks will be contributing on a regular basis to International Atomic Time as secondary representations of the second, improving its stability and accuracy. Since international timescales underpin modern-day technology such as telecommunications and navigation systems, this work will benefit end users across a wide range of sectors in the longer term.