Count the working days between two dates, UK bank holidays included

This is arithmetic until it is a promise. The moment a date calculation appears in a service level agreement, a notice period, a statutory deadline or an escalation clock, being a day out stops being an inconvenience and starts being a breach of something you signed.

Working days are where it goes wrong, because the term is doing more work than it looks. Working days exclude weekends and bank holidays, and UK bank holidays are not one list: England and Wales, Scotland and Northern Ireland differ, and some of them move. A contract that says five working days without saying whose calendar has left a gap that will eventually be argued about, usually at the least convenient moment.

The other recurring trap is whether you count the day you start on. Five working days from Monday is Friday if you count Monday and the following Monday if you do not, and both readings appear in real agreements. It is worth settling explicitly in the wording rather than discovering the disagreement when a deadline is missed by exactly one day.

For support and service teams this arithmetic underpins the numbers everyone reports on. A first response target measured in working hours behaves very differently from one measured in elapsed time, particularly across a bank holiday weekend, and a team can hit its target comfortably while a customer waits four days. Both figures are true. Only one of them describes the experience.

That gap is worth looking at deliberately rather than accepting. If your reported performance is healthy while the complaints suggest otherwise, the measurement is usually the reason, and the fix is agreeing what the clock is supposed to represent rather than adjusting the target.

Everything happens in this page, which for this applet is less about confidentiality and more about not depending on somebody else's service to answer a question about a contract you are in the middle of.