Add/subtract working days, count business days between two dates, or test if a date is a business day — with YOUR holiday list. No account, no API key, one GET. Built so AI agents get deterministic date math (LLMs are unreliable at it).
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", ... }
Limitations
v0.1 is pure UTC date arithmetic — date-only, no time-of-day, no DST/timezones (that is recurrence-rule territory).
You supply the holidays — no built-in country holiday database (caller passes a holidays= list; that is deliberate).
Open source — self-host or use it free.
Pricing
Free — Anonymous · no signup · no key · 120 requests / 10 min per IP ·
pure UTC date math. The free tier never changes.
Founder Key — $19 once, lifetime — 20× rate limit
(2400 req / 10 min) · priority best-effort · supports the project.
Pay $19 in crypto (BTC / ETH / USDC / many others) — instant, no
card, no signup. You receive an API key string. One-time payment,
no subscription — pass the key as ?key=
or x-api-key: header.