Why counting business days is harder than it looks

"Delivery in 10 business days." Sounds simple, right up until two people land on two different dates. Here are the traps I keep running into, plus a small JavaScript algorithm that doesn't drift by a day.

1. Does the start date count?

It's Monday and the deadline is "5 business days". Is that this Friday or next Monday? It depends on the convention:

Exclusive (the start date doesn't count): Monday + 5 business days = next Monday.
Inclusive (the start date is day 1): the 5th business day is Friday.

Excel uses both. WORKDAY returns the date a given number of working days after the start date, so the start date itself isn't counted. NETWORKDAYS counts whole working days including both ends. So NETWORKDAYS(start, WORKDAY(start, 5)) returns 6 when the start is a workday. Agree on the convention before you argue about the deadline.

2. Weekends aren't always Saturday and Sunday

A few examples:

Saudi Arabia: Friday–Saturday
Israel (Friday–Saturday for most offices; Sunday is a workday)
Retail, healthcare and shift teams often have a single day off, or a rotating one

Excel handles this with WORKDAY.INTL and NETWORKDAYS.INTL. The weekend argument takes a number (1 = Saturday–Sunday, 7 = Friday–Saturday, 11 = Sunday only) or a 7-character string that starts on Monday, where 1 means day off: "0000011" is Saturday–Sunday, "0000001" is Sunday only.

If your code hard-codes [0, 6], it's wrong for a lot of people.

3. Holidays move

Observed holidays. For US federal employees, a holiday that falls on a Saturday is observed on the Friday before, and one that falls on a Sunday is observed on the Monday after.
Substitute holidays. The UK and South Korea, among others, add a substitute day off when certain holidays land on a weekend.
Local holidays. National, state and city holidays stack. There is no single "holiday list" for a country that works for everyone.

The right holiday list is the one that applies to that team, in that place, this year.

4. Exceptions: a day off that becomes a workday

Make-up days, inventory Saturdays, on-call weekends... A usable model needs both extra days off and exceptional working days, and the exceptions have to override the weekly day off.

5. Time zones break dates

In JavaScript, new Date("2026-03-02") is parsed as midnight UTC. Anywhere west of UTC (the Americas, for example), getDay() and getDate() give you Sunday the 1st instead of Monday the 2nd. Parse date-only strings yourself and stay in UTC from start to finish.

An algorithm that handles all of the above

const parse = s => { const [y, m, d] = s.split("-").map(Number); return new Date(Date.UTC(y, m - 1, d)); }; const fmt = d => d.toISOString().slice(0, 10); function isWorkday(date, { weekendDays = [0, 6], holidays = [], extraWorkdays = [] } = {}) { const key = fmt(date); if (extraWorkdays.includes(key)) return true; // an exception beats the weekly day off if (holidays.includes(key)) return false; return !weekendDays.includes(date.getUTCDay()); // 0 = Sunday } // n > 0 moves forward, n < 0 moves backward. // inclusive: the start date counts as day 1 if it is a workday. function addWorkdays(start, n, { inclusive = false, ...cal } = {}) { const step = Math.sign(n) || 1; let left = Math.abs(n); const d = parse(start); if (left === 0) return fmt(d); if (inclusive && isWorkday(d, cal)) left--; while (left > 0) { d.setUTCDate(d.getUTCDate() + step); if (isWorkday(d, cal)) left--; } return fmt(d); } const cal = { holidays: ["2026-11-20"] }; addWorkdays("2026-11-16", 5, cal); // "2026-11-24" addWorkdays("2026-11-16", 5, { ...cal, inclusive: true }); // "2026-11-23" addWorkdays("2026-03-02", 5); // "2026-03-09" addWorkdays("2026-03-02", 5, { inclusive: true }); // "2026-03-06"

It walks one calendar day at a time instead of doing clever math with / 7. For deadlines measured in days or weeks, that's plenty fast, and it's much easier to explain why a given date was skipped.

In TypeScript, I'd type the calendar as { weekendDays: number[]; holidays: string[]; extraWorkdays: string[] } and use Sets if the lists get long.