Work package leader: Rodolphe Le Targat, OBSPARIS Other participants: GUM, INRIM, LNE, NPL, PTB, POLITO, SRC PAS, UMK The aim of this work package is to realise prototype physical timescales based on optical clock data. The international reference timescale Coordinated Universal Time (UTC) is highly stable, taking into account contributions from over 400 clocks around the world. However, it is a paper timescale, disseminated by the BIPM bulletin Circular T, with a latency of 15 – 40 days. Local physical realisations of UTC (denoted UTC(k), where k stands for laboratory k) are therefore maintained by NMIs and DIs to provide real-time references from which time and frequency signals are broadcast to end users via a variety of dissemination techniques. Methods will be demonstrated for incorporating optical clocks into these UTC(k) timescales, building on preliminary paper studies performed within the SIB55 ITOC project. Compared to previous work in this direction, the use of robust optical clocks will enable an operating mode that is less reliant on the availability of hydrogen masers with the lowest possible instabilities. Key to this will be the development of software routines for automated real-time validation of the experimental data, across the whole metrological chain connecting the optical clocks to the hydrogen masers generating the timescales. The steering methods employed in our experimental prototypes will be informed by numerical simulations which take into account realistic operational parameters and constraints. The stability of the experimental timescales will be evaluated by direct comparison as well as by comparison to standard UTC(k) timescales and to UTC.