JSON API

Two endpoints, no key, no rate limit: the current model offset between lunar and Earth clocks, and the drift accumulated between any two dates.

What this is

Every number this API returns is a model value. No lunar time scale has been realized, and a coordinate time scale is not read by any real clock, so the API returns what the published rate figures predict — not a reading taken on the Moon. ^[1]Official The arithmetic is the same code that drives the clock on this site, and it is written out in full on the methodology page.

Because of that, every response carries three fields you cannot switch off: disclaimer,methodology and sources. If you show these numbers to someone else, show those fields too.

Endpoints

The version lives in the path, and the same version is repeated in the version field of every response. All responses are application/json; charset=utf-8, all times are UTC, and CORS is open: access-control-allow-origin: *, so a browser on any origin can call this directly.

GET /api/v1/now

Example

Request:

curl https://lunartime.org/api/v1/now

Response, shortened — the model array holds one entry per rate figure, andsources one entry per source those figures rest on:

{
  "api": "lunartime.org",
  "version": "v1",
  "generated_utc": "2026-08-24T13:43:01.584Z",
  "disclaimer": "Model values. No lunar time scale has been realized. See the methodology page.",
  "methodology": "https://lunartime.org/clock/methodology/",
  "utc": {
    "iso": "2026-08-24T13:43:01.584Z",
    "unix_ms": 1787578981584
  },
  "tt": {
    "iso": "2026-08-24T13:44:10.768Z",
    "offset_from_utc_s": 69.184
  },
  "model": [
    {
      "id": "surface-vs-geoid",
      "label": "Lunar surface clock vs Earth geoid clock",
      "faster": "clock near the Moon's selenoid",
      "slower": "clock near Earth's geoid",
      "rate_us_per_day": 56.02,
      "epoch": "1977-01-01T00:00:00.000Z",
      "elapsed_days": 18132.572,
      "offset_us": 1015786.66,
      "offset_s": 1.01578666,
      "offset_formatted": "+1.015787 s",
      "seconds_per_year": 0.0204613,
      "years_per_second": 48.87,
      "sources": [
        "ashby-patla-2024",
        "our-calculation-model"
      ]
    }
  ],
  "constants": {
    "tai_minus_utc_s": 37,
    "tt_minus_tai_s": 32.184,
    "epoch_tcl_1977": "1977-01-01T00:00:00.000Z",
    "epoch_j2000": "2000-01-01T11:58:55.816Z"
  },
  "sources": [
    {
      "id": "ashby-patla-2024",
      "title": "A Relativistic Framework to Estimate Clock Rates on the Moon",
      "issuer": "Neil Ashby and Bijunath R. Patla, NIST — The Astronomical Journal 168:112",
      "url": "https://iopscience.iop.org/article/10.3847/1538-3881/ad643a",
      "verified": "2026-08-23"
    }
  ],
  "license": "Free to use. Attribution requested: link to https://lunartime.org/"
}

GET /api/v1/drift

Deterministic: the same input always gives the same answer, which is why this endpoint is cached for an hour. A to earlier than from is not an error — the result is negative and the sign is shown.

Example

Request:

curl "https://lunartime.org/api/v1/drift?from=1977-01-01&to=2026-08-23"

Response:

{
  "api": "lunartime.org",
  "version": "v1",
  "input": {
    "from": "1977-01-01T00:00:00.000Z",
    "to": "2026-08-23T00:00:00.000Z",
    "rate_us_per_day": 56.02,
    "rate_id": "surface-vs-geoid"
  },
  "result": {
    "days": 18131,
    "microseconds": 1015698.62,
    "seconds": 1.01569862,
    "formatted": "+1.015699 s",
    "years_per_second": 48.87
  },
  "pair": {
    "faster": "clock near the Moon's selenoid",
    "slower": "clock near Earth's geoid"
  },
  "disclaimer": "Model values. No lunar time scale has been realized. See the methodology page.",
  "methodology": "https://lunartime.org/clock/methodology/",
  "sources": [
    "ashby-patla-2024",
    "our-calculation-model"
  ],
  "license": "Free to use. Attribution requested: link to https://lunartime.org/"
}

Errors

Errors are JSON too, never an empty body and never HTML. The shape is always{"error":{"code":"…","parameter":"…","message":"…"}}, andparameter appears only when the error is about a parameter.

Using it

• Free, no key, no quota. Free to use. Attribution requested: link to https://lunartime.org/ If traffic from one caller starts to hurt the site we will limit it, and this page will say so before we do.
• Attribution. Where the number is shown, link to the page it came from — themethodology field of every response is that link.
• Say that it is a model. The disclaimer field is written to be displayed, not stripped. A microsecond figure without its pair of scales, or without the word model, misleads the reader in exactly the way this site exists to prevent.

Version policy

A breaking change gets a new path — /api/v2/ — and v1 keeps answering. Adding a field is not a breaking change, so parse responses leniently: ignore fields you do not know rather than failing on them. Removing or renaming a field, changing the meaning of one, or changing an error code is breaking, and will not happen inside a version.