What counts as a business day
A business day is any calendar day that is (1) not a weekend day and (2) not a holiday from your list. Both parts are configurable, because “the weekend” is not universal: the default Saturday–Sunday pattern does not hold in much of the Middle East (Friday–Saturday) or for six-day weeks, and public holidays differ by country, region and even employer.
Two different questions
“Business days” answers two distinct questions, and mixing them up produces off-by-one errors:
- Count between dates: “how many working days does this project span?” The convention used here: count from the day after the start date through the end date, inclusive. Monday → Friday in one week is 4 business days; Monday → the following Monday is 5.
- Add N business days: “what date is 10 working days from now?” Day 0 is the start date itself (when it is a business day); day 1 is the next business day; and so on.
A worked example
Take Monday 2026-10-26 → Monday 2026-11-02 with a Sat–Sun weekend:
- Calendar days elapsed (start exclusive, end inclusive): 7.
- Weekend days in the span: 2 (Sat + Sun).
- Business days: 5 (Tue, Wed, Thu, Fri, Mon).
Now add one holiday on Thursday 2026-10-29: the business-day count drops to 4, while the calendar count stays 7. This is exactly why holiday lists belong next to the calculation, and why a calculator that silently assumes some country’s holiday calendar can mislead you.
Why not divide by seven
Shortcuts like “calendar days × 5/7” are approximations that drift with the start weekday and holidays. The reliable method walks the calendar day by day and classifies each one, which is what the Business Days Calculator does (with a scan cap of about a thousand years so an absurd input cannot freeze the page).
Daylight saving does not matter here
Business days are counted on the calendar, not in milliseconds. Even though some local days are 23 or 25 hours long around a clock change, every date still exists exactly once. Dividing a duration by 86,400,000 (the millisecond-per-day shortcut) is the mistake that makes such counts go wrong across DST boundaries.
Checklist for a defensible count
- State the convention (start excluded or included) next to the number.
- Configure the weekend to match the organization, not the default.
- Load the real holiday list for the applicable jurisdiction, including moveable ones for the years you span.
- For payroll-grade rounding, keep gross and net hours separate (see the Work Hours Calculator); durations and business days are different questions.