Space organizations are starting to consider how to keep time on the Moon, which is a major undertaking necessary to underpin the growing number of lunar missions and settlements planned for the coming decades. Currently, NASA and ESA are planning to install satellite navigation systems to support the communication and navigation processes required for reliable and accurate real-time positioning. To maximize interoperability and facilitate cross-organizational endeavours, these systems should employ the same timescale. An internationally accepted lunar reference time would set a common framework for all lunar systems and users. Nevertheless, stable timekeeping on the Moon raises technical issues. Due to the different gravitational field, a clock on the Moon will not tick at the same rhythm as on the Earth; therefore, even the most perfectly synchronized clocks would quickly gain a considerable offset. This raises the necessity to settle aspects such as:

• Should lunar time be set on an independent basis, or should it be regularly synchronized with the UTC ?
• Should the atomic clock on the lunar surface or in orbit be the reference time scale ?
• How can clocks on the Moon be measured and converted into UTC in order to be compared with clocks on Earth ?

In its role as responsible for the realization and dissemination of the UTC and for the definition of measurement units and reference time scales, the BIPM was invited to join the international forum discussing this topic, together with the International Astronomical Union which fixes the conventional reference frames.