Date arithmetic looks trivial and is not. These are longer explanations of the problems that catch people out — written to be read on their own, whether or not you use the calculators.
A calculator gives you a number. It does not tell you whether the number is the one your situation calls for — whether your contract means calendar days or business days, whether a due date is a midpoint or a deadline, whether the weekday you just looked up for a 1740 parish record is the weekday the parish clerk would have recognised.
Every guide here started as a question we could not answer in a sentence inside a tool page. They are updated when the underlying facts change — time zone rules and public-holiday calendars both move more often than people expect — and each carries the date it was last reviewed.
If something here is wrong or out of date, we would like to know: get in touch.