How we calculate
The formulas behind each calculator, how dates and rounding are handled, what the tests cover, and what the site deliberately leaves out.
One method per tool, written down
Each calculator implements a single, stated method. Where a question has more than one reasonable convention — counting days inclusively, for example — the page shows both and explains the difference, instead of silently picking one.
Dates are calendar dates, not timestamps
Date maths is done on calendar dates in UTC. Two consequences:
- A daylight-saving change between your two dates cannot add or remove a day.
- Month lengths are real. Someone born on 31 January reaches “one month later” on 28 February, not 3 March, and 29 February is handled explicitly rather than rolling into March.
Invalid dates such as 31 February are rejected. They are never quietly converted.
Rounding
Calculations run at full precision and round only for display, usually to two decimal places. We never round an intermediate value, because rounding twice is how a total ends up a cent away from the sum of its parts.
What the tests cover
Every calculator has unit tests that run before a deploy; a failure blocks the release. They cover the cases that actually break calculators:
- month-end borrowing and leap days
- a range crossing a daylight-saving boundary
- zero and negative input, and division by zero
- reverse tax (removing 20% from 120 must give 100, not 96)
- overnight shifts and breaks longer than the shift
- mpg to litres per 100 km, checked against a known result
What we deliberately leave out
Some things vary too much to hard-code honestly:
- Public holidays. Working-day counts exclude weekends only. A wrong holiday table is worse than none.
- Overtime multipliers, tips and shift premiums. The hours calculator returns hours and gross pay at one rate.
- Local tax and benefit rules. These get their own country page with a rule-year badge, or they are not published at all.
If a number looks wrong
Send the inputs, the result you expected and the result you got. Corrections are made on the page and the review date moves with the change. If a page cannot be made correct, it is taken down rather than left up with a caveat.