Guides
Explanations of the rules and conventions behind the calculators — the parts where two reasonable people can get different answers, and where knowing which convention applies matters more than the arithmetic.
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How working days are calculated
What counts as a working day, how substitute bank holidays differ between countries, and where counting starts when you add working days rather than count them.
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Understanding ISO week numbers
Why week 1 is defined by the first Thursday, why 1 January can fall in week 53 of the previous year, and which years have 53 weeks.
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How daylight saving affects calculations
Instants against wall-clock times, the hour that never happens, the hour that happens twice, and why recurring international meetings drift twice a year.
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How to calculate hours worked
Night shifts, paid against unpaid breaks, decimal hours, and why writing 7 hours 30 minutes as 7.30 quietly costs twelve minutes.
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Adding months to a date, and month-end clamping
Why one month after 31 January is 28 February, why the arithmetic is not reversible, and which convention this site follows.
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Unix timestamps and ISO 8601
Telling seconds from milliseconds by digit count, why timestamps have no time zone, the Year 2038 problem, and the boundaries worth testing against.
Why the conventions matter more than the maths
Very few date and time questions are hard to compute. Almost all of them are hard to specify. How many days are there between Monday and Friday? Four, or five, depending on whether you are measuring elapsed time or counting a working week. When is a leap-year baby's birthday in a non-leap year? Depends on the jurisdiction. What time is 01:30 on the morning the clocks go back? There are two answers, an hour apart.
Most calculators pick one interpretation silently and present the result as fact. The approach here is to make the choice visible: every calculator states which convention it applied, lets you change it where a reasonable alternative exists, and flags the cases where the answer is genuinely ambiguous rather than papering over them.
Where the data comes from
- Public holidays — computed from each country's published rules rather than stored as a fixed table, so the calendar stays correct for future years. National holidays only.
- Time zones — the IANA time zone database built into your browser, the same source used by operating systems and most programming languages.
- Calendar arithmetic — the proleptic Gregorian calendar and ISO 8601 week rules throughout.
More guides to come
Standalone articles on notice periods, payroll date rules, holiday entitlement and public holiday reference tables by country are planned. If there is something you keep having to look up, let us know.