Julian Day Calculator API

GET

Convert between calendar dates and continuous Julian Day numbers.

Julian Day Calculator converts between Gregorian calendar dates and Julian Day Numbers (JDN). Julian Day is a continuous count of days used in astronomy and historical research.

Try it — live request, no key required

Request

GETapi.apiverve.com/v1/julianday?date=2024-03-15

Query parameters

datestring

Verification

Format

No key required to try it. Get a key to use it in your app.

Example

{
  "status": "ok",
  "error": null,
  "data": {
    "gregorian": "2024-03-15",
    "julianDayNumber": 2460385,
    "modifiedJulianDate": 60384.5,
    "dayOfWeek": "Friday",
    "weekNumber": 11,
    "dayOfYear": 75,
    "daysFromToday": -340
  }
}

About the Julian Day Calculator API

Perfect for astronomical calculations, historical date research, and applications that need a continuous day count across calendar systems.

What people use it for

Astronomical Software: Calculate precise day numbers for celestial events
Historical Research: Convert historical dates to a unified day count
Date Arithmetic: Calculate day differences across long time spans
Scientific Applications: Use continuous day counts for time series data

Ways to call it

One endpoint, many ways in — REST with JSON, XML, YAML and CSV, plus GraphQL and an MCP interface for AI agents.

JSON: Default REST response
XML: Markup format
YAML: Human-readable
CSV: Tabular export
GraphQL: Query language
MCP: For AI agents

View all integrations →

Other ways to use Julian Day Calculator

Same data, same APIVerve account, same credit balance — one key works on all of them.

Give it to an AI agentConnect over MCP and your agent calls it as a native tool — Claude, Cursor, ChatGPT.on VerveKit Google Sheets or ExcelA =VERVE() formula fills a column — no script, no export, recalculates in place.on VerveSheets Ground an agent on itA cited, machine-checkable fact your model can't produce on its own.on VerveContext

Questions.

Common questions about the Julian Day Calculator API.

Read the docs →

Which plan fits 50,000 date conversions a month?

The Starter plan covers that volume. Each conversion uses 2 credits, so 50,000 calls consume 100,000 of the 200,000 credits available on Starter, working out to about $0.30 per 1,000 requests. The Free plan provides 100 conversions every month for testing.

Is the days-from-today offset included on the Free plan?

No, the relative days from today calculation is a premium field. The Free plan returns standard Julian Day Numbers, Modified Julian Dates, and day-of-year figures, while upgrading to any paid plan unlocks the daysFromToday count.

Can I convert in both directions between calendar dates and Julian days?

Yes, the calculator supports bidirectional conversion. You can convert a Gregorian date into its Julian Day Number or provide a Julian Day Number to get the matching calendar year, month, and day.

Does it calculate Modified Julian Date as well?

Yes, both endpoints return the Modified Julian Date alongside the standard Julian Day Number. Responses also calculate useful calendar markers, including the ISO week number, day of the week, and day of the year.

What format is required for Gregorian date inputs?

Gregorian dates must be formatted strictly as YYYY-MM-DD strings. For reverse conversions, the Julian Day Number must be provided as a whole integer.

Request parameters — GET /v1/julianday

Sent in the query string. Premium parameters are accepted on every plan but only take effect on plans that include them. Anything not listed here is dropped rather than passed through.

Required

Parameter	Type	Example	Description
`date`Required	string	`2024-03-15`	The Gregorian date in YYYY-MM-DD format date

Example request

1
2

curl "https://api.apiverve.com/v1/julianday?date=2024-03-15" \
  -H "x-api-key: YOUR_API_KEY"

Authentication

Send your key in the x-api-key header. That is the only auth step — no token exchange, and no per-endpoint scope to configure.

Header	When	Value
`x-api-key`Required	Every request	Your API key. Header names are case-insensitive, so `X-API-Key` is the same header.
`Authorization`Alternate	Instead of the above	`Bearer <key>` — for clients that only expose bearer auth. Resolves to the same account and the same billing.

401 is the only auth verdict

A 401 means the key is missing, invalid or expired. A 403 means the key is valid but not permitted here — blocked by a key restriction or an IP allow-list. Running out of credits is a 429.

Response — GET /v1/julianday

Every APIVerve endpoint returns the same three top-level keys, so one response handler covers your whole integration: status, error and data. Only data changes shape.

Sample response

{
  "status": "ok",
  "error": null,
  "data": {
    "gregorian": "2024-03-15",
    "julianDayNumber": 2460385,
    "modifiedJulianDate": 60384.5,
    "dayOfWeek": "Friday",
    "weekNumber": 11,
    "dayOfYear": 75,
    "daysFromToday": -340
  }
}

Response fields

Paths are relative to data. Premium fields are absent rather than zeroed on plans that do not include them, so check for presence instead of comparing to 0.

Field	Type	Example	Description
`gregorian`	string	`"2024-03-15"`	The equivalent Gregorian calendar date
`julianDayNumber`	number	`2460385`	Days elapsed since noon on 1 January 4713 BC
`modifiedJulianDate`	number	`60384.5`	Julian day number shifted to start at midnight on 17 November 1858
`dayOfWeek`	string	`"Friday"`	Day of the week the date falls on
`weekNumber`	number	`11`	ISO week number (1-53)
`dayOfYear`	number	`75`	Day of the year (1-366)
`daysFromToday`Premium	number	`-340`	Number of days from today (negative = past, positive = future)

Response headers

Every response carries your balance, so your own code always knows where it stands without polling anything. All four are exposed to browsers through CORS.

Header	What it carries
`x-api-remaining-credits`	Credits left in the current cycle
`x-api-credits-used`	Credits spent in the current cycle
`x-api-max-credits`	The allowance for the cycle
`x-api-version`	Version of the endpoint that answered

Errors

Read the HTTP status first, then error for the specific reason. The body names the parameter that has to change.

Status	Meaning	What to do
`400`	Input was rejected	Read `error`; it names the parameter that has to change.
`401`	Key missing or invalid	Check the header name and the key value.
`403`	Key valid, but not permitted here	A key restriction or an IP allow-list — never a bad key.
`429`	Rate limited, or out of credits	Read `error` to tell them apart, then back off or top up.

What a call costs — 2 credits per call

Credits are shared across every APIVerve API — one balance, one key, one bill. Failed requests never count against it, so a rejected input costs nothing.

Plan	Per month	Credits	Julian Day Calculator calls	Per 1,000 calls
Free	Free	200	100	Free
Starter	$29.99	200,000	100,000	$0.30
Pro	$99.99	1,000,000	500,000	$0.20
Mega	$299.99	4,000,000	2,000,000	$0.15

Calls are what the credits buy at this API's rate — spend them here, on any of the other APIs, or across both. Nothing is reserved per endpoint.

Compare all pricing tiers →

Call it from cURL — GET /v1/julianday

No SDK to install — this is a plain HTTPS call to /v1/julianday with your key in the x-api-key header. Swap in a real key and it runs as-is.

cURL

1
2

curl "https://api.apiverve.com/v1/julianday" \
  -H "x-api-key: YOUR_API_KEY"

Resources

GitHubapiverve/julianday-apiSource, issues and the examples this page is built from.ReferenceJulian Day Calculator API docsEvery parameter, every response field, and the errors.ParametersWhat this call acceptsRequired and optional inputs, with example values.

Also published for

@apiverve/juliandaynpm APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from Node.js — GET /v1/julianday

Install the Node.js SDK, then call the Julian Day Calculator API with your key. The client wraps the same HTTPS request, so anything it returns is what the endpoint returns.

Install — npm

npm install @apiverve/julianday

Node.js

const res = await fetch("https://api.apiverve.com/v1/julianday", {
  headers: { "x-api-key": "YOUR_API_KEY" },
});
const { data } = await res.json();
console.log(data);

Resources

npm@apiverve/juliandayThe published Node.js client, versioned and signed.GitHubapiverve/julianday-apiSource, issues and the examples this page is built from.ReferenceJulian Day Calculator API docsEvery parameter, every response field, and the errors.ParametersWhat this call acceptsRequired and optional inputs, with example values.

Also published for

APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from Python — GET /v1/julianday

No SDK to install — this is a plain HTTPS call to /v1/julianday with your key in the x-api-key header. Swap in a real key and it runs as-is.

Python

import requests

res = requests.get(
    "https://api.apiverve.com/v1/julianday",
    headers={"x-api-key": "YOUR_API_KEY"},
)
print(res.json()["data"])

Resources

Also published for

@apiverve/juliandaynpm APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from C# / .NET — GET /v1/julianday

Install the C# / .NET SDK, then call the Julian Day Calculator API with your key. The client wraps the same HTTPS request, so anything it returns is what the endpoint returns.

Install — NuGet

dotnet add package APIVerve.API.JulianDayCalculator

C# / .NET

using var client = new HttpClient();
client.DefaultRequestHeaders.Add("x-api-key", "YOUR_API_KEY");

var json = await client.GetStringAsync("https://api.apiverve.com/v1/julianday");
Console.WriteLine(json);

Resources

NuGetAPIVerve.API.JulianDayCalculatorThe published C# / .NET client, versioned and signed.GitHubapiverve/julianday-apiSource, issues and the examples this page is built from.ReferenceJulian Day Calculator API docsEvery parameter, every response field, and the errors.ParametersWhat this call acceptsRequired and optional inputs, with example values.

Also published for

@apiverve/juliandaynpm

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from Go — GET /v1/julianday

No SDK to install — this is a plain HTTPS call to /v1/julianday with your key in the x-api-key header. Swap in a real key and it runs as-is.

req, _ := http.NewRequest("GET", "https://api.apiverve.com/v1/julianday", nil)
req.Header.Set("x-api-key", "YOUR_API_KEY")

res, _ := http.DefaultClient.Do(req)
defer res.Body.Close()
body, _ := io.ReadAll(res.Body)
fmt.Println(string(body))

Resources

Also published for

@apiverve/juliandaynpm APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from PHP — GET /v1/julianday

No SDK to install — this is a plain HTTPS call to /v1/julianday with your key in the x-api-key header. Swap in a real key and it runs as-is.

PHP

$ch = curl_init("https://api.apiverve.com/v1/julianday");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, ["x-api-key: YOUR_API_KEY"]);

$response = curl_exec($ch);
echo $response;

Resources

Also published for

@apiverve/juliandaynpm APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

Call it from Ruby — GET /v1/julianday

No SDK to install — this is a plain HTTPS call to /v1/julianday with your key in the x-api-key header. Swap in a real key and it runs as-is.

Ruby

require "net/http"

uri = URI("https://api.apiverve.com/v1/julianday")
req = Net::HTTP::Get.new(uri)
req["x-api-key"] = "YOUR_API_KEY"

res = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) { |h| h.request(req) }
puts res.body

Resources

Also published for

@apiverve/juliandaynpm APIVerve.API.JulianDayCalculatorNuGet

Only the languages APIVerve ships a client for are listed — the rest call the endpoint directly over HTTPS.

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Julian Day Calculator API

Try it — live request, no key required

About the Julian Day Calculator API

What people use it for

Ways to call it

Other ways to use Julian Day Calculator

Questions.

Request parameters — GET /v1/julianday

Required

Authentication

Response — GET /v1/julianday

Response fields

Response headers

Errors

What a call costs — 2 credits per call

Call it from cURL — GET /v1/julianday

Resources

Also published for

Call it from Node.js — GET /v1/julianday

Resources

Also published for

Call it from Python — GET /v1/julianday

Resources

Also published for

Call it from C# / .NET — GET /v1/julianday

Resources

Also published for

Call it from Go — GET /v1/julianday

Resources

Also published for

Call it from PHP — GET /v1/julianday

Resources

Also published for

Call it from Ruby — GET /v1/julianday

Resources

Also published for

Zero-code embed — one snippet, no backend

Changelog — Julian Day Calculator API

Science

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