lunartime.org

Lunar time: a timeline from 1991 to 2028

Lunar timekeeping has no single starting point. This is the chain of documents that leads to it, in order, with the primary source on every entry.

Key facts

  • The IAU General Assembly adopted Resolutions II and III in August 2024, defining the Lunar Celestial Reference System and Lunar Coordinate Time. [1]Official
  • The OSTP memorandum of 2 April 2024 named Coordinated Lunar Time and set 31 December 2026 as the date for NASA to deliver a finalized strategy. [2]Official
  • ESA made the public case for a common lunar reference time on 27 February 2023, after agencies met at ESTEC in November 2022. [3]Official
  • The 28th CGPM is scheduled for 13 to 15 October 2026, with lunar time on the provisional agenda as Draft Resolution D. [4]Official

The first section holds what has already taken place. The second holds dates that documents announce and that nobody has yet reached. Nothing moves from the second section to the first until a source says it happened.

Rate figures appear in several entries below — the microsecond-per-day differences between clocks on the Moon and clocks on Earth. Each of those is a model result published in the paper that is cited beside it, not a reading taken from a clock on the Moon. How we use those figures is set out in how we calculate lunar time, and the figures themselves drive the lunar clock model. For where each organisation stands today, rather than how it got there, see the lunar time standard tracker. How every entry below is sourced, and how we correct one when it turns out to be wrong, is set out in about lunartime.org.

The last two scheduled entries below turn on one meeting. What is on its agenda, and what each outcome would change, is covered article by article in the lunar time news hub.

This page grows one entry at a time, and an entry is added only when a document lands. Those additions are also what the email brief reports: the Lunar Time Brief goes out when there is something worth reporting, at most weekly.

Happened

Each entry is dated as precisely as its source dates it, and cites that source. Where a source gives only a month or a year, only a month or a year is shown.

  1. IAU defines the barycentric and geocentric reference systems

    The IAU adopted the barycentric and geocentric reference systems in 1991; the 2024 lunar resolution builds on that work and cites it. [1]Official

  2. IAU 2000 fixes the BCRS/GCRS metric and the TCB–TCG relation

    The IAU 2000 resolutions fixed the metric of the barycentric and geocentric systems. Resolution II of 2024 reuses that construction for the Moon, with lunar quantities in place of terrestrial ones. [1]Official

  3. Space agencies meet at ESTEC and agree a common lunar reference time is needed

    At a meeting at ESA's ESTEC centre in November 2022, agencies agreed on the importance and urgency of defining a common lunar reference time. [3]Official

  4. ESA publishes "Telling time on the Moon"

    ESA described the ESTEC discussion publicly and set out why lunar missions need a common reference time rather than each mission keeping its own. [3]Official

  5. OSTP issues the Celestial Time Standardization policy

    The memorandum named Coordinated Lunar Time (LTC), required traceability to UTC and set three separate deadlines for NASA. [2]Official[5]Official

  6. Kopeikin and Kaplan publish "Lunar Time in General Relativity"

    The paper sets out a general-relativistic framework for lunar time, grounded in the IAU 2000 resolutions. [6]

  7. NIST publishes the relativistic framework for lunar clock rates

    Ashby and Patla estimated how fast clocks run on the lunar surface and at the Lagrange points relative to a clock on Earth's geoid: about 56 microseconds per day. [7]Official[8]

  8. IAU General Assembly adopts Resolutions II and III on LCRS/TCL

    Resolution II defines the Lunar Celestial Reference System and Lunar Coordinate Time; Resolution III asks international organizations to agree how a lunar reference time scale relates to UTC. [1]Official[9]Official

  9. NASA states it will develop a lunar time standard

    NASA said it would work with international partners on a lunar time standard for LunaNet, citing a difference of about 56 microseconds per day against a clock on Earth. [10]Official

  10. OSTP issues the Lunar Reference Systems policy

    The companion memorandum covers lunar reference systems and asks NASA for an implementation plan no later than 31 December 2026. [11]Official

  11. ICG and IOAG hold their first joint cislunar PNT workshop

    The UN committee on GNSS and the Interagency Operations Advisory Group met on 11 to 13 February 2025 for the first of their joint multilateral workshops on cislunar positioning, navigation and timing. [12]Official

  12. CCTF reports work on a lunar reference time scale toward the CGPM

    The CCTF reported work with space agencies on a lunar reference time scale, and workshops with lunar system providers ahead of the conference. [13]Official

  13. LTE440 lunar time ephemeris published in Astronomy & Astrophysics

    LTE440 converts between lunar and terrestrial time scales with an accuracy better than 0.15 ns before 2050. [14]

  14. BIPM circulates the CGPM convocation listing Draft Resolution D

    The convocation lists Draft Resolutions A to F for the 28th CGPM. Lunar time is Draft Resolution D and agenda item 17. [4]Official

  15. Metrologia publishes "Lunar reference timescale"

    Bourgoin, Defraigne and Meynadier set out a lunar reference timescale and its relation to TCG, with about 56 microseconds per day between the two surfaces. [15]

  16. Second joint ICG-IOAG cislunar PNT workshop meets in Vienna

    The second joint workshop ran in Vienna from 10 to 13 February 2026. Its page names lunar reference systems and time systems as the focus and records ICG Working Group L on lunar PNT. [12]Official

  17. NASA restructures Artemis: Artemis III becomes a 2027 low-Earth-orbit test

    NASA added a mission and updated the architecture: Artemis III becomes a low-Earth-orbit test in 2027, and the first crewed landing moves to Artemis IV in 2028. [16]Official

Scheduled

Nothing below has happened. These are dates that documents announce, and an announced date can move. Each one stays in this section until a source says it took place.

  1. Scheduled

    28th CGPM meets in Versailles and votes on Draft Resolution D

    The 28th CGPM is scheduled for 13 to 15 October 2026 at the Palais des Congres de Versailles, where Draft Resolution D is on the provisional agenda. [17]Official[4]Official

  2. Scheduled

    NASA's finalized lunar timing standardization strategy is due to the Executive Office of the President

    The 2024 memorandum sets 31 December 2026 as the date by which NASA is to provide the finalized implementation strategy. [2]Official[5]Official

  3. Scheduled

    NASA's lunar reference systems implementation plan is due

    The companion memorandum sets the same date, 31 December 2026, for the lunar reference systems implementation plan. [11]Official

  4. Scheduled

    Artemis IV: first crewed lunar landing under the updated architecture

    Under the architecture published in 2026, the first crewed landing is planned for Artemis IV in 2028. [16]Official

Sources

  1. Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL) — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .
  2. Policy on Celestial Time Standardization — White House Office of Science and Technology Policy, . Official document. Verified .
  3. Telling time on the Moon — European Space Agency, . Official document. Verified .
  4. Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001) — Bureau International des Poids et Mesures / CIPM, . Official document. Verified .
  5. White House Office of Science and Technology Policy Releases Celestial Time Standardization Policy — White House Office of Science and Technology Policy, . Official document. Verified .
  6. Lunar Time in General Relativity — Sergei M. Kopeikin and George H. Kaplan — Physical Review D 110, 084047, . Peer-reviewed. Verified .
  7. What Time Is It on the Moon? — National Institute of Standards and Technology, . Official document. Verified .
  8. A Relativistic Framework to Estimate Clock Rates on the Moon — Neil Ashby and Bijunath R. Patla, NIST — The Astronomical Journal 168:112, . Peer-reviewed. Verified .
  9. Resolution on the establishment of a coordinated lunar time standard by international agreement — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .
  10. NASA to Develop Lunar Time Standard for Exploration Initiatives — NASA, . Official document. Verified .
  11. Policy on Standardization of Lunar Reference Systems in Support of the National Cislunar Science & Technology Strategy — White House Office of Science and Technology Policy, . Official document. Verified .
  12. 2nd Joint ICG-IOAG Multilateral Cislunar PNT Workshop — Interagency Operations Advisory Group (IOAG) with the UN ICG, . Official document. Verified .
  13. Latest Updates from 2025 CCTF Meeting — Bureau International des Poids et Mesures, . Official document. Verified .
  14. Lunar Time Ephemeris LTE440: definitions, algorithm and performance — Xu Lu, Tian-Ning Yang, Yi Xie, Purple Mountain Observatory, Chinese Academy of Sciences — Astronomy & Astrophysics 704, A76, . Peer-reviewed. Verified .
  15. Lunar reference timescale — A Bourgoin, P Defraigne, F Meynadier — Metrologia 63(1) 015003, . Peer-reviewed. Verified .
  16. NASA Adds Mission to Artemis Lunar Program, Updates Architecture — NASA, . Official document. Verified .
  17. 28th meeting of the CGPM (2026) — Bureau International des Poids et Mesures, . Official document. Verified .

Last verified