Date Difference Calculator

The off-by-one that costs money

Whether a date range includes its endpoints is the single most consequential detail in date arithmetic, and it is settled by convention rather than by logic. A hotel booked from the 1st to the 5th is four nights, not five, because you leave on the morning of the 5th. A four-day leave request starting Monday covers Monday to Thursday inclusive. An interest period of thirty days may count the first day or the last but conventionally not both. Every one of these is an inclusive-versus-exclusive decision, and getting it wrong produces a result that is off by exactly one — which is small enough to slip through review and large enough to cause a genuine dispute about a bill, a contract or an entitlement. Always establish which convention applies before you count.

Working days are a different question entirely

A great many practical deadlines are expressed in working days, and that count depends on things a calendar does not know. Weekends must be excluded, which is straightforward. Public holidays must also be excluded, and those vary by country, by region within a country, and by year — English bank holidays differ from Scottish ones, and both shift around Easter. Some organisations observe their own closure days. This is why 'ten working days' means genuinely different calendar dates depending on where you are and when you start, and why a legal or contractual deadline expressed in working days should always be checked against the specific holiday calendar that governs it rather than assumed.