---
title: "Calendar conversion to Julian Day Number"
method: POST
path: "/time/julian-day"
tags: ["Time Conversion"]
---

# Calendar conversion to Julian Day Number

`POST /time/julian-day`

## Request

- Base URL: `https://api.openephemeris.com`
- URL: `https://api.openephemeris.com/time/julian-day`
- 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`

Bridge between civil calendars and astronomical time. Handles BCE dates.

## Headers

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

## Request body

- JulianDayRequest
  - `calendar` string: Calendar system to use (e.g. proleptic_gregorian, julian)
  - `day` number, required: Day of the month, can include fractional days
  - `hour` number: Hour of the day (0-23)
  - `minute` number: Minute of the hour (0-59)
  - `month` integer, required: Month of the year (1-12)
  - `second` number: Second of the minute (0-59)
  - `time_scale` string: Time scale (utc, tt)
  - `year` integer, required: Astronomical year (0 = 1 BCE, -1 = 2 BCE)

## Response `200`

Successful Response

- JulianDayResponse
  - `input_interpretation` object, required: Metadata about the input date/calendar
  - `julian_day` number, double, required: Julian Day number
  - `julian_day_terrestrial` number, double, required: Julian Day in Terrestrial Time (TT)
  - `modified_julian_day` number, double, required: Modified Julian Day (JD - 2400000.5)
  - `valid_range` object, required: Validity range for high-precision calculations

## 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)