Why did adding a holiday not change the answer?

Because it fell on a weekend or outside the range, so it was never in the working total. Subtracting it would make the result a day short. The tool counts how many of your dates were ignored and says so, rather than quietly getting it wrong in either direction.

Should I count both ends?

It depends what you are measuring. For how many days a task takes, count both: Monday to Friday is five working days. For how long until a deadline, leave it off: Monday to the following Monday is five working days away.

Which days are the weekend?

Saturday and Sunday. That is the working week in most of the world but not all of it, and there is no way to tell from a date which convention applies. If your weekend falls elsewhere, list those days as holidays.

Business Days Calculator

Working days between two dates, holidays taken off.

Runs in your browserNothing you enter is uploaded, and it keeps working with the network off.

From

Count both endsYes

Holidays

Working days between two dates, holidays taken off.

How to use it

Give the two dates. Both as YYYY-MM-DD, which is the only way to write a date without ambiguity.
Decide about the ends. Off counts the gap between them. On counts both days as worked.
Paste your holidays. One per line. Only weekday holidays inside the range change the answer.

When you would use this

Counting working days is subtraction with two complications, and both of them are where the wrong answers come from. The first is which ends count. Monday to Friday is five working days if you are asking how long a task takes and four if you are asking how far away Friday is. Neither is more correct, so it is a switch rather than an assumption, and the result says which it used. The second is holidays. A holiday only changes the answer if it falls on a weekday inside the range. Christmas Day 2026 is a Friday, so it takes a day off that week. Boxing Day is a Saturday, and subtracting it would make the answer a day short, because it was never counted as worked. The same goes for a date outside the range and for the same date listed twice. All three are filtered out, and the number of dates that changed nothing is reported so you can see the filter worked rather than assuming it did. A range that runs backwards gives a negative count, which is the honest answer to a backwards question.

Questions

Why did adding a holiday not change the answer?: Because it fell on a weekend or outside the range, so it was never in the working total. Subtracting it would make the result a day short. The tool counts how many of your dates were ignored and says so, rather than quietly getting it wrong in either direction.
Should I count both ends?: It depends what you are measuring. For how many days a task takes, count both: Monday to Friday is five working days. For how long until a deadline, leave it off: Monday to the following Monday is five working days away.
Which days are the weekend?: Saturday and Sunday. That is the working week in most of the world but not all of it, and there is no way to tell from a date which convention applies. If your weekend falls elsewhere, list those days as holidays.