---
title: "Delta T (TT-UT1) calculation"
method: GET
path: "/time/delta-t"
tags: ["Time Conversion"]
---

# Delta T (TT-UT1) calculation

`GET /time/delta-t`

## Request

- Base URL: `https://api.openephemeris.com`
- URL: `https://api.openephemeris.com/time/delta-t`
- Auth: one of:
  - HTTP bearer
  - API key in header `X-OpenEphemeris-API-Key`
  - API key in header `X-API-Key`
  - API key in header `X-Service-Key`

Provide Delta T (TT - UT1) for any historical/future epoch.

## Query parameters

- `date` string, required: Date (YYYY-MM-DD)

## Headers

- `authorization` string, nullable
- `X-API-Key` string, nullable
- `X-OpenEphemeris-API-Key` string, nullable
- `apikey` string, nullable

## Response `200`

Successful Response

- DeltaTResponse
  - `date` string, required: Date string used for calculation
  - `delta_t` object, required: Delta T value (seconds) and its components
  - `julian_day` number, double, required: Julian Day for which Delta T was calculated
  - `model` string, required: The mathematical model used (e.g., Stephenson 2021)
  - `uncertainty` object, required: Estimated uncertainty in the Delta T value

## Other responses

- `400`: Bad Request
- `422`: Validation Error
- `500`: Internal Server Error

---

[API](https://skmtc.dev/openephemeris/apis/open-ephemeris-api.md) · [All operations](https://skmtc.dev/openephemeris/apis/open-ephemeris-api/llms.txt) · [OpenAPI document](https://skmtc.dev/openephemeris/apis/open-ephemeris-api/revisions/d195a769cb89?raw)