A deadline is a piece of arithmetic before it is anything else, and the arithmetic is not intuitive. Whether the first day counts, whether Saturdays and holidays are inside the period or outside it, whether a month means thirty days or the same date next month, and what happens when the last day falls on a closure are all answered by rules that differ between forums. This subject sets out how a period is actually counted and where the counting goes wrong.
Some periods run through weekends and holidays and some skip them, and the distinction is usually drawn by the length of the period or stated expressly in the rule. Getting it wrong on a short period is the single most common counting error, because the wrong convention produces a plausible date rather than an obviously absurd one.
