Openworkdays MCP server

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

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.

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

(max 1000). Strictly validated.

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

Errors

Errors are JSON with an error key:

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

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

• 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.

application/json response — no sessions, no SSE)

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

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

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

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:

Shortened. The full README is on GitHub.