A 90 day window contains about 64 working days once 26 weekend days are removed. At 7.6 hours a day that is 486.4 working hours, or 12.80 working weeks.
Take out public holidays and any leave, and the gap between the calendar figure and the available figure widens further.
Two different units with the same name
Contracts, delivery estimates and statutory notice periods are quoted sometimes in business days and sometimes in calendar days, often without saying which. The two only coincide for very short periods, and even then only if no weekend intervenes.
Five business days is a week. Five calendar days is five days. Over a longer window the ratio settles at roughly five sevenths, so a 30 day figure is about 21 working days and a 90 day figure about 64.
Where it goes wrong in planning
The common failure is estimating effort in working days and then setting a deadline in calendar days, or the reverse. A task estimated at 60 working days does not finish 60 calendar days from now; it finishes about 84 calendar days out, before holidays.
The error is systematic rather than random, which is why it compounds across a plan instead of averaging away. Every task inherits it in the same direction.
Holidays are the part that varies
Public holidays differ by country, and within some countries by state or region, so a schedule that works for one office does not automatically work for another. A period spanning a major holiday season can lose a working week that a naive calculation never sees.
This is why the business days calculator takes a holiday count as an input rather than assuming a calendar. Nothing about the holiday schedule is stable enough to hard-code, and the business days in a month calculator answers the narrower question for a single month.
Which unit to quote
For anything involving other people's time, quote business days and say so explicitly. For anything involving elapsed time, such as interest, spoilage, a cooling-off period or a warranty, calendar days are the honest unit, because the world does not stop at the weekend.
Interest is the clearest case: a loan accrues every calendar day regardless of whether anyone is at work, which is why compounding frequency is counted in calendar periods rather than working ones. For a simple countdown, the days until calculator works in calendar days.