OpenWorkdays

A zero-signup business-day / working-day date-arithmetic API. No account, no API key — one GET request.

Add or subtract working days, count business days between two dates, or test whether a date is a business day — with your own holiday list. Pure UTC date math, returned as JSON. Built so autonomous AI agents get a deterministic date endpoint — an agent has no human to do a signup, and LLMs are measurably unreliable at date arithmetic.

Live: https://openworkdays.vercel.app

Quick start

curl "https://openworkdays.vercel.app/api/businessdays?start=2026-05-15&days=5"

Sample response:

{
  "mode": "add",
  "start": "2026-05-15",
  "days": 5,
  "result": "2026-05-22",
  "resultWeekday": "fri",
  "startIsBusinessDay": true,
  "weekend": ["sat", "sun"],
  "holidaysApplied": 0,
  "engine": "date-utc-v0.1",
  "computedAt": "2026-05-17T00:00:00.000Z"
}

Any error returns JSON with an error key and an appropriate HTTP status (400 bad params, 429 rate limited, 405 wrong method).

The three modes

The endpoint is always GET /api/businessdays. The mode is inferred from the params you pass.

add — add/subtract working days

Params: start=YYYY-MM-DD, days=N (integer, +/- ok, |N| <= 10000), optional weekend=, optional holidays=.

curl "https://openworkdays.vercel.app/api/businessdays?start=2026-05-15&days=5"

N=0 returns start unchanged (even if start is itself non-working).

diff — count business days between two dates

Params: start=YYYY-MM-DD, end=YYYY-MM-DD, optional weekend=, optional holidays=. Inclusive of both endpoints. Result is negative if end < start.

curl "https://openworkdays.vercel.app/api/businessdays?start=2026-05-15&end=2026-06-15&holidays=2026-05-25"

is — is this date a business day?

Params: date=YYYY-MM-DD, optional weekend=, optional holidays=.

curl "https://openworkdays.vercel.app/api/businessdays?date=2026-05-16"

Returns isBusinessDay plus a reason of "weekend", "holiday", or null.

Shared params

• `weekend` (default sat,sun) — comma list. Tokens are case-insensitive

from {sun,mon,tue,wed,thu,fri,sat}, or numeric 0-6 where 0=Sunday … 6=Saturday (matches JS getUTCDay()). Cannot mark all 7 days as weekend.

• `holidays` — comma list of YYYY-MM-DD dates treated as non-working

(max 1000). Strictly validated.

Errors

Errors are JSON with an error key:

• 400 — bad/missing params (missing start, invalid days (must be integer),

invalid weekend token, invalid holiday date, invalid date, specify a mode (includes a usage object), …)

• 429 — rate limited (Retry-After header + retryAfterSec)
• 405 — wrong HTTP method (only GET/OPTIONS allowed)

JavaScript

const base = "https://openworkdays.vercel.app";
const r = await fetch(
  base + "/api/businessdays?start=2026-05-15&days=5"
);
const out = await r.json(); // { mode:"add", result:"2026-05-22", resultWeekday:"fri", ... }

Use as an MCP tool (AI agents / Claude / Cursor / LLM clients)

OpenWorkdays is also a remote MCP server, so an agent can call it as a tool with no signup, no API key, no OAuth.

• Endpoint: `https://openworkdays.vercel.app/api/mcp`
• Transport: Streamable HTTP, stateless (POST JSON-RPC 2.0, single

application/json response — no sessions, no SSE)

• Exposes exactly one tool: `businessdays` — the mode (add / diff /

is) is inferred from which args you pass, returning the same JSON as the REST endpoint (also as structuredContent)

Drop this into any MCP client config (Claude Desktop, Cursor, or any client that speaks the Streamable HTTP transport):

{ "mcpServers": { "openworkdays": { "url": "https://openworkdays.vercel.app/api/mcp" } } }

Quick smoke test:

curl -s -X POST https://openworkdays.vercel.app/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"businessdays","arguments":{"start":"2026-05-15","days":5}}}'

Over MCP, an upstream error comes back as a tool result with isError: true.

Why

Every public business-day API gates behind an account or API key: workingdays.org is test-only / subscription; API Ninjas and Holiday API need keys; timeanddate is a $299 product; the RapidAPI business-day APIs need a RapidAPI key; Microsoft's "Calculate Working Day" connector is Power-Platform-framework-gated and UK-bank-holidays only. OpenWorkdays is a single anonymous GET — no signup, no key. That is especially useful to autonomous agents, which have no human in the loop to complete a signup. And because LLMs are measurably unreliable at date arithmetic, an agent needs a deterministic endpoint rather than guessing.

Limitations

• Date-only, pure UTC arithmetic — no time-of-day, no DST, no timezones

(that is recurrence-rule territory, intentionally out of scope for v0.1).

• Caller-supplied holidays only — there is no built-in country holiday

database. This is deliberate: we do not fake-compete on the 7000-holiday-DB axis where funded incumbents win. You pass the holidays= list you care about.

• Best-effort per-instance IP rate limit — a soft abuse brake, not a

guarantee (serverless instances are ephemeral and not shared).

• MIT licensed and self-hostable — see below.

Self-host — it's zero-dependency files

The entire API is two single zero-dependency Node serverless functions: api/businessdays.js (REST) and api/mcp.js (remote MCP). No npm install, no deps at all. Deploy the folder to Vercel (zero-config /api detection) or drop the handlers into any Node serverless runtime.

git clone https://github.com/SolvoHQ/openworkdays
cd openworkdays
npx vercel --prod

License

MIT — see LICENSE.