ForHosting KIT · Developer Utilities

Doomsday rule calculator

John Conway's Doomsday rule is a mental algorithm that finds the weekday of any Gregorian calendar date by first locating the year's anchor day—the so-called doomsday that falls on memorable dates such as April fourth, June sixth, and December twelfth—and then offsetting from that anchor to the day you care about.

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

This doomsday rule calculator applies the same century-anchor and year-offset arithmetic deterministically, so students, tutors, and pipelines get a structured weekday answer without installing calendar libraries or trusting the host clock. Send a single ISO date string such as twenty twenty-four dash zero seven dash zero four; the response names the weekday, reports Conway's zero-through-six index, the year's anchor day, the century anchor, and the month doomsday day used in the offset. Impossible calendar dates like a non-leap February twenty-ninth fail with a clear invalid input error. The browser widget runs free on your device; the prepaid API charges $0.002 per successful item when servers and SDKs need the same contract.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/date/doomsday-weekday

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/date/doomsday-weekday \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"date":"2024-07-04"}'

const res = await fetch("https://api.kit.forhosting.com/date/doomsday-weekday", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "date": "2024-07-04"
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/date/doomsday-weekday",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "date": "2024-07-04"
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/date/doomsday-weekday", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"date":"2024-07-04"}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"date":"2024-07-04"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/date/doomsday-weekday", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "date": "2024-07-04"
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "date.doomsday_weekday",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →

Run — free