They're not going to be maintaining literal atomic clocks on the Moon for this. They'll apply a mathematical adjustment to UTC based on what the physics calculations say is happening. The details of that adjustment are what NASA has yet to develop. It could involve subtracting a "leap second" from lunar time at intervals, leap seconds are already used for keeping UTC in sync with the solar time so it's an established process. Or maybe they'll just let Lunar time continue drifting relative to Earth, in which case there'll be a different "epoch" on each.