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Small tools for the parts of life here that come down to a rule and some arithmetic — the questions people ask most and get wrong most. Each one shows the rule it implements and links the official source, so you can check it rather than trust it. They are free to use, link to and quote.

Why these exist

Both answer a question that has a published, checkable answer and that almost everybody gets wrong in the same direction. The tax one exists because arrival and departure days do not count and people assume they do. The insurance one exists because your employer chooses your tier and most people never find out which.

If one gives you a wrong answer, tell me: [email protected].