What is Coordinated Lunar Time (LTC)?

Coordinated Lunar Time, abbreviated LTC, is a proposed time standard for the Moon. It is meant to serve lunar operations the way Coordinated Universal Time serves Earth. The name comes from a United States policy memorandum; the physics comes from work published separately.

This page collects what has been proposed, who proposed it, and which questions nobody has answered yet.

Coordinated Lunar Time does not exist yet

There is no clock anywhere that reads LTC. No agency computes it, no bulletin publishes it, and no document defines its realization. What exists is a name, a deadline, and a coordinate time from which such a scale could be built.

That distinction matters for every number you will read about lunar time. Our own clock shows a model value: arithmetic on a published rate figure, not a reading from an instrument. The clock methodology writes out the formula in full.

What has actually been proposed

Four bodies have put something on the table. They are not proposing the same object, and only one of them has actually voted.

A second IAU resolution of the same assembly asks international organizations to agree how a lunar reference time scale, a lunar coordinate time and UTC relate to each other ^[6]Official. That request is still open.

LTC, TCL and lunar time are not the same thing

Three names circulate, and the press mixes them freely. They mean different things.

• LTC is the operational scale named in the 2024 policy memorandum: the scale lunar missions would actually use, kept traceable to UTC ^[2]Official.
• TCL is a coordinate time, defined by the IAU in 2024 as the time coordinate of the Lunar Celestial Reference System ^[3]Official. No real clock reads a coordinate time directly ^[6]Official.
• Lunar time is the everyday umbrella term. It carries no definition at all, which is why it appears in headlines rather than in documents.

An operational scale is built on top of a coordinate time. So LTC would be derived from TCL, not a competitor to it. Where both of them sit among the older scales is drawn in the family tree of time scales.

Why the Moon needs its own scale

A clock resting on the lunar surface does not tick at the rate of a clock resting on Earth’s geoid. The published secular difference is 56.02 µs per day in favour of the lunar clock ^[5]. NIST states the same result in round numbers, about 56 microseconds per day for a lunar surface clock against an Earth clock ^[7]Official.

Microseconds sound harmless until they are converted into distance. Light travels 299 792 458 m in one second, so one microsecond of timing error is 299.79 m of light path ^[1]. Navigation on the Moon has to do better than that.

Running lunar operations on Earth time would therefore mean carrying a growing correction in every position fix. The article on why clocks run faster on the Moon takes the two physical effects apart, and why microseconds matter on the Moon converts the rates into metres of ranging error.

Who decides

No single body owns this question. Four are involved, and they move on different clocks of their own.

• The CGPM, the diplomatic conference of the metre convention, meets from 13 to 15 October 2026 in Versailles, with lunar time on the provisional agenda ^[8]Official. Its preparatory committee, the CCTF, has been working with space agencies on a lunar reference time scale ^[9]Official.
• The IAU has already acted: it defines the reference system and the coordinate time ^[3]Official.
• NASA holds a national deadline rather than an international mandate: a finalized implementation strategy is due no later than 31 December 2026 ^[2]Official.
• The ICG, through its working group on lunar positioning, navigation and timing, is where agencies compare notes in public ^[10]Official.

Our standards tracker records where each of them stands today and what the next dated milestone is. The recipe those bodies would be copying is set out in how UTC is calculated.

What is still open

• The realization. A definition is not a time scale. Somebody has to run clocks, weight them and publish a result on a schedule. No agency has announced that it will do so; how a lunar time scale would be realized lists every layer that is missing.
• The tie to UTC. IAU Resolution III asks international organizations to settle the relations between a lunar reference time scale, a lunar coordinate time and UTC ^[6]Official. That agreement does not exist yet.
• One scale or several. The CIPM has recorded that a single lunar reference time scale is needed even if it has several realizations ^[11]Official. What a realization would look like in practice is undecided. The article on time zones on the Moon goes through the proposals.
• Who keeps the clocks. European agencies agreed in 2022 that a common lunar reference time was urgent, without saying who would maintain it ^[12]Official.

None of this is a prediction. It is a list of decisions that have not been taken, and we will change this page when they are. The rest of the Learn hub covers the physics behind the proposals and the questions readers ask most often.

One question comes up before all of them: whether this subject is the same as counting days and dates. It is not, and why lunar time is not a calendar sets out the difference.

Last verified 24 August 2026