Methodology

These calculators put dates on deadlines that people act on — filings, notices, invoices, leave, settlement. That only works if you can see where the numbers come from and how they are checked. This page describes both, including the failures that shaped the process.

Where the rules come from

Every calculator that implements a legal or regulatory period cites its primary source on its own page, under Where these rules come from. The rule is to link the statute, the regulation or the agency that administers it — the eCFR, the US Code, EUR-Lex, the New Zealand Legislation website — rather than a summary, a law-firm article or another calculator. Where a period is contractual rather than statutory, as with SLA targets or net payment terms, the page says so plainly instead of implying an authority that does not exist.

A tool with no citation is a tool whose rule we have not verified to that standard, and it renders no sources section at all. An empty heading would be worse than an honest gap, and a confident-looking wrong citation would be worse than both.

How the holiday data is maintained

Public holidays are computed, not stored as a flat list, because a stored list goes stale silently. Fixed-date holidays, nth-weekday holidays and Easter-derived holidays each have their own generator, and the observance rules — Mondayisation in New Zealand, substitute days in the UK, the weekend rollover in the US federal calendar — are applied on top.

Three specific corrections are worth stating because they were real errors found in this data, and they explain why parts of it look more careful than they need to:

• Christmas and Boxing Day move as a pair. Shifting them independently made them collide on the same date, and in a date-keyed map one silently overwrote the other — quietly deleting a public holiday from Canada 2026, New Zealand and Australia 2027, and the UK 2027. The invariant now asserted after any change is that every UK, NZ, AU and CA December yields exactly two weekday holidays, every year from 2024 to 2060.
• Victoria Day is the Monday on or before 24 May, not the fourth Monday in May. The fourth-Monday shortcut is wrong in seven years out of every twelve.
• Matariki cannot be computed. It is lunar, and the New Zealand dates are set by a gazetted schedule. The table holds that schedule to 2052 and the tool warns beyond it. A first attempt at extending it from memory diverged from the real dates at 2036, which is exactly why it is now checked against the Act rather than recalled.

Exchange calendars are kept separate from civil ones. Settlement and trading-day calculations use the NYSE and NZX calendars — which include Good Friday and exclude Columbus Day and Veterans Day — because a market holiday and a federal holiday are not the same set, and using the civil calendar for settlement produces dates that are wrong in both directions.

Coverage is deliberately uneven and labelled where it is. India ships only the three gazetted national holidays, and the country picker says so on the page rather than letting you discover it from a count that runs high.

How the calculators are tested

Every calculator's compute function is exercised against edge-case inputs — empty fields, zero, negative numbers, reversed date ranges, extreme years — and the checks inspect what a visitor would actually see rather than internal state. Several classes of bug are now asserted against directly because each of them shipped once:

• A blank field is not a zero. Reading numbers with the common parseInt(x) || default idiom cannot tell an empty input from a deliberate 0. It reinstated a waived real-estate financing contingency, turned "Net 0" into Net 30, and swallowed midnight as a valid start hour. Thirty-eight of those were replaced with a parser that distinguishes the two.
• Every declared input must actually render. A field with no renderer used to vanish silently — which is how a shift-rotation calculator shipped with no pattern selector while every automated check passed. A missing renderer now produces a visible error.
• Business-day counts must be given a real holiday set. Several tools passed an empty one and quietly counted federal holidays as working days.
• List-building tools clamp both the loop and the input. An invoice scheduler asked for 9,999 cycles emitted nearly 20,000 rows and hung the browser.

How the writing is produced

Each calculator page carries a hand-written explanation of the rule it implements, a worked example using real dates, and answers to questions that are actually asked about it. Two rules govern that writing. The first is that content follows code: explanations are written against the calculator's own logic, never from its title or tagline, and a compute change and its content update ship together. If the page and the code disagree, that is a bug in one of them and it is treated as one.

The second is that worked examples are verified against the real holiday data rather than written from memory. Four examples were wrong on a first pass because Juneteenth falls inside a June window and New Zealand's King's Birthday fell on 1 June that year. Every weekday and date claim across the calculator pages and the 118 articles on the blog is re-checked by script after any content change, and all of them must resolve.

What we do not do

• We do not publish a calculator without a written explanation of the rule behind it.
• We do not fill a state or jurisdiction table with a default. Tools with partial coverage return no date for an uncovered jurisdiction, because a plausible wrong date is worse than a blank — a default here once reported personal-injury deadlines a year late in one-year states.
• We do not present an estimate as a deadline. Processing-time tools say they are estimates from published averages.
• We do not give legal, tax, financial or medical advice, and these tools are not a substitute for a professional who knows your facts.

Corrections

Wrong results are the reports we act on fastest. If a calculator returns a date you can show is wrong, tell us which tool, the exact inputs including the holiday calendar and workweek, the date it returned, the date you expected, and the rule you are counting against. Those details separate a holiday-calendar mismatch from a genuine counting bug immediately. Report it here.

Each calculator page shows the date its rule set was last verified. That date changes when the rule is re-checked against its source, not when the wording around it is edited.

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