Why holidays are deliberately left out

A universal "business day" calculator that also knew every regional public holiday would need a maintained holiday calendar per country, and often per state or region within a country — US federal holidays differ from UK bank holidays, which differ again from holidays observed in any other jurisdiction, and they shift from year to year (Easter-linked holidays, for instance, don't fall on a fixed date). Rather than ship an inevitably incomplete or wrong holiday list, this tool sticks to the one part of "business day" that's actually universal — Monday through Friday — and leaves holiday exclusion to whoever's using it, who knows their own relevant calendar far better than a generic tool could guess at.

The inclusive/exclusive convention, and why it matters for SLAs

Counting business days between two dates inclusively on one end and exclusively on the other matches the common real-world phrasing "how many business days until X" — the start date itself is where you're standing, not a day you're counting, while the end date is the deadline being counted toward. This convention shows up constantly in SLA and deadline calculations ("ticket must be resolved within 3 business days of filing"), and getting the inclusive/exclusive boundary backwards is a common source of off-by-one errors when someone re-implements this logic by hand rather than using a tool that's already settled on one consistent convention.