Why "Business Days" Needs More Than Just Subtracting Weekends
Calculating the number of working days between two dates isn't simply the total day count minus weekends — the actual weekday/weekend pattern within a given date range depends on exactly which days of the week the start and end dates fall on, so two ranges of the identical total length can have different business-day counts depending on where each range's weekend days happen to land.
The naive shortcut of "total days times 5/7" only works as a rough average over long ranges — it's measurably inaccurate for shorter ranges, where the specific position of Saturdays and Sundays within that particular window matters far more than a simple ratio can capture. Getting this exactly right requires actually walking through the date range day by day (or using a closed-form formula that correctly accounts for the starting day-of-week), not applying a blanket average.
Public holidays are a separate consideration this kind of calculator typically doesn't account for by default, since holiday calendars vary by country, region, and even specific organization — a genuinely "working days excluding holidays" calculation needs an explicit holiday list layered on top of the weekend-exclusion logic, not something that can be assumed universally.