ForHosting KIT · Developer Utilities

Reverse due date calculator

This reverse due date calculator estimates the conception date from a given estimated due date by subtracting 266 calendar days.

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

That offset is the classic mean interval from fertilization to a 280-day last-menstrual-period pregnancy clock: forty weeks from a notional LMP minus about fourteen days from LMP to ovulation and conception. You enter a real Gregorian due date in YYYY-MM-DD form; the engine validates the calendar day, including month lengths and leap days, then walks the civil calendar backward without using the JavaScript Date object, network calls, or randomness. The same pure module powers the free browser run on this page and the API, so homework keys, prenatal-app fixtures, and CI goldens match every time for the same due date. Failed validation is not billed; each successful API estimate costs $0.002. Treat the result as an educational and engineering estimate, not a clinical diagnosis or a substitute for professional obstetric judgment.

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

What does the reverse due date calculator cost?

It is free in your browser on this page. Through the API a successful request costs $0.002; invalid due dates are not charged.

Why subtract 266 days instead of 280?

A standard obstetric pregnancy is counted as 280 days from last menstrual period. Mean conception is about 14 days after LMP, so the mean fertilization-to-EDD interval is 266 days. This tool inverts that clock.

What date format does due_date accept?

A real Gregorian calendar day in YYYY-MM-DD form, for example 2026-10-15. Month names, slashes, times of day, and impossible days such as 2024-02-30 are rejected as invalid_input.

Does the result include an LMP estimate as well?

Yes. Besides conception_date the response includes lmp_estimate equal to the due date minus 280 days, plus the fixed offsets days_subtracted (266) and pregnancy_length_days (280).

Is this medical advice or a diagnostic tool?

No. The result is an educational and engineering estimate from a fixed 266-day mean interval. Clinical dating decisions need professional obstetric interpretation, cycle history, and often ultrasound.

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/health/reverse-due-date

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/health/reverse-due-date \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"items":["valor-1","valor-2"]}'

const res = await fetch("https://api.kit.forhosting.com/health/reverse-due-date", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "items": [
      "valor-1",
      "valor-2"
    ]
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/health/reverse-due-date",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "items": [
        "valor-1",
        "valor-2"
    ]
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/reverse-due-date", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"items":["valor-1","valor-2"]}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"items":["valor-1","valor-2"]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/reverse-due-date", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "items": [
    "valor-1",
    "valor-2"
  ]
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "health.reverse_due_date",
  "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.

Limits

`min_year`	1
`max_year`	9999
`days_subtracted`	266

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 →