There's currently no answer to that question, but with plans in place to inhabit the Moon, that may need to change.
At a recent meeting in the Netherlands, members from space organizations around the world agreed that we need to implement a proper lunar time zone – an internationally accepted common lunar reference time that all future missions can use to communicate and navigate with ease.
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"A joint international effort is now being launched towards achieving this," says navigation system engineer Pietro Giordano from the European Space Agency (ESA).
The recent Netherlands meeting was hosted and led by researchers at ESA, but the discussion was extremely collaborative.
The goal is to put together a mutually agreed-upon framework, called the LunaNet, which will provide a common interface for all future lunar missions, streamlining how they network, navigate, detect, inform, and communicate.
Timing will be key for those future operations.
In the next few years, several robotic landers will be sent off to the Moon from various space organizations and private companies. What's more, ESA, NASA, the Japan Aerospace Exploration Agency (JAXA), and the Canadian Space Agency (CSA) are working together on establishing an orbiting lunar station, called Gateway, where future expeditions can launch from.
"These missions will not only be on or around the Moon at the same time, but they will often be interacting as well – potentially relaying communications for one another, performing joint observations or carrying out rendezvous operations," reads a press release from the ESA.
Historically, every mission that has gone to the Moon has used the atomic clocks on Earth to track their progress, synchronizing their time in space with their time on Earth.
This basically requires 'radioing home' and asking people on Earth what time it is, while also accounting for the time it takes to make that call.
A normal old clock on board a spacecraft simply won't do the trick. The forces of gravity and velocity are different on the Moon, which means they impact time in different ways than the forces nearer our own planet.
Practically, this means that if a lunar astronaut brought a watch with them from Earth, it would run faster than normal by dozens of microseconds a day. How fast depends on whether that astronaut is in orbit or standing on the Moon itself.
Under these complex conditions, stable timekeeping set specifically to the Moon will be tricky to establish, but it could be more accurate and faster than synchronizing with Earth time.
That's what scientists are discussing right now. Do we stick to Earth time or move to lunar time?
The latter scenario will require putting together a working lunar time system and a common coordinate system for the surface of the Moon, like what we use on Earth to track orbiting satellites.
This may take more energy and effort, but it could result in a much more accurate system – one which could then be applied to other planets as well.
"Of course, the agreed time system will also have to be practical for astronauts," explains Bernhard Hufenbach, head of strategic planning at ESA.
"This will be quite a challenge on a planetary surface where in the equatorial region each day is 29.5 days long, including freezing fortnight-long lunar nights, with the whole of Earth just a small blue circle in the dark sky."
It's a mathematician's dream puzzle.
"Throughout human history, exploration has actually been a key driver of improved timekeeping and geodetic reference models," says Javier Ventura-Traveset, who is coordinating ESA contributions to LunaNet.
"It is certainly an exciting time to do that now for the Moon… "
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