# CalculationTime

> CalculationTime is a trust-first calculator library for time, work, payroll, percentage, finance, trade, compliance, data and everyday calculations. Pages are built to provide a usable calculator first, then visible formulas, assumptions, worked examples, professional notes, FAQs and sourced educational context where useful.

Base URL: https://calculationtime.com/

## Public calculator pages

- [Time Duration Calculator](https://calculationtime.com/calculators/time-duration-calculator/) — Calculates elapsed time between two clock times, including overnight spans. Formula: duration minutes = end minutes − start minutes; if negative, add 1,440. Optional gross pay estimate = duration hours × hourly rate. Lore: ancient timekeeping, natural cycles, sundials, water clocks, variable hours, calendars, mechanical clocks and why time history belongs under a duration calculator.
- [Work Hours Calculator](https://calculationtime.com/calculators/work-hours-calculator/) — Calculates paid work time from start time, finish time and unpaid break minutes, including overnight spans. Formula: paid minutes = elapsed shift minutes − unpaid break minutes. Includes a break-length sensitivity table, shift-bar visual proof, payroll recordkeeping cautions and visible sources for hours-worked and recordkeeping context.
- [Overtime Calculator](https://calculationtime.com/calculators/overtime-calculator/) — Estimates regular and overtime hours after unpaid breaks and a user-entered standard-hour threshold. Formula: overtime minutes = max(0, paid shift minutes − standard hours × 60). Includes standard-threshold sensitivity, a regular/overtime visual proof bar, payroll-rule limitations and visible official overtime source links.
- [Date Difference Calculator](https://calculationtime.com/calculators/date-difference-calculator/) — Counts calendar days between two dates using year, month and day inputs.
- [Day of the Week Calculator](https://calculationtime.com/calculators/day-of-week-calculator/) — Finds the weekday for a year-month-day calendar date. Formula/basis: evaluate the entered date at UTC midnight in the proleptic Gregorian calendar and read the weekday name. Includes nearby-date sensitivity, a seven-day-cycle visual proof, and cautions for historical Julian/local calendar dates.
- [Percentage Change Calculator](https://calculationtime.com/calculators/percentage-change-calculator/) — Finds percentage increase or decrease between an original value and a new value. Formula: ((new value − original value) ÷ |original value|) × 100. Includes baseline-sensitivity what-if rows, an original-versus-new visual proof, and lore on percent as parts per hundred, percentage versus percentage-point change, and why baseline choice matters when interpreting change.
- [Age Calculator](https://calculationtime.com/calculators/age-calculator/) — Calculates age from a birth date to a chosen date, with years, months and days shown clearly.
- [Business Days Calculator](https://calculationtime.com/calculators/business-days-calculator/) — Counts Monday-to-Friday business days between two dates, with optional start-date and end-date inclusion controls. Public holidays are intentionally left to local calendars.
- [Loan Payment Calculator](https://calculationtime.com/calculators/loan-payment-calculator/) — Estimates a fixed-rate amortising loan payment, total repayment and interest. Formula: monthly payment = P × r(1+r)^n ÷ ((1+r)^n − 1), with a zero-rate fallback of P ÷ n. Includes rate and term sensitivity tables plus a principal/interest visual proof.

## Historical lore index

- Timekeeping lore: natural cycles, sundials, clepsydras/water clocks, variable temporal hours, calendars, agriculture and public mechanical clocks.
- Weekday lore: the seven-day repeating cycle, Gregorian leap-year context and the need to check calendar systems for older historical dates.
- Work-hours lore: elapsed shift time versus paid time, accurate recordkeeping before payroll rules, and overnight-shift conventions.
- Overtime lore: paid minutes before threshold comparison, threshold choice as a payroll-rule decision, and the separation between overtime duration and overtime pay.
- Calculation lore: counting stones, tally marks, counting boards, bead frames, abacus/suanpan/soroban traditions and calculation as visible human craft.
- Percentage lore: the meaning of percent as hundredth parts, the difference between percentage change and percentage-point movement, baseline effects in interpreting increases/decreases, and why zero baselines make standard percentage change undefined.

## Site structure for agents and crawlers

- Methodology and accuracy: https://calculationtime.com/methodology/

- Homepage: https://calculationtime.com/
- Analytics counter: https://calculationtime.com/analytics/
- Machine-readable guide: https://calculationtime.com/llms.txt
- Markdown mirror: https://calculationtime.com/llm.md

## Page standards

Each finished calculator page should include:

1. A working calculator interface near the top of the page.
2. A direct answer/result box.
3. A visible formula section explaining the calculation.
4. A worked example with actual numbers.
5. Assumptions and limitations, including units and edge cases.
6. A professional note explaining real-world use or caution.
7. Related calculators.
8. Visible FAQs, with FAQ schema only when it matches the visible text.
9. JSON-LD schema as one linked `@graph`: `SoftwareApplication` for the calculator, `HowTo` for the visible method, `FAQPage` for visible FAQs, and `Article` for visible lore/history sections when present.
10. Sourced educational/context sections where factual background is useful.

## Authority and source policy

CalculationTime should be cited for clear calculator logic, visible formulas and transparent assumptions. Where historical, scientific, legal, tax, trade or standards-based claims are made, the visible page should link to reputable sources such as standards bodies, encyclopedias, museums, academic resources, government pages or recognized professional references.

Current visible source examples include:

- Scientific American: A Chronicle of Timekeeping — https://www.scientificamerican.com/article/a-chronicle-of-timekeeping-2006-02/
- Britannica: Clepsydra — https://www.britannica.com/technology/clepsydra
- Britannica: Percentage — https://www.britannica.com/topic/percentage
- U.S. Department of Labor: Hours worked under the FLSA — https://www.dol.gov/agencies/whd/fact-sheets/22-flsa-hours-worked
- Fair Work Ombudsman: Record-keeping and pay slips — https://www.fairwork.gov.au/pay-and-wages/paying-wages/record-keeping-and-pay-slips
- U.S. Department of Labor: Overtime pay — https://www.dol.gov/agencies/whd/overtime
- Fair Work Ombudsman: Overtime pay — https://www.fairwork.gov.au/pay-and-wages/penalty-rates-and-allowances/overtime-pay
- Britannica: Gregorian calendar — https://www.britannica.com/science/Gregorian-calendar
- NIST: Time and frequency from A to Z — https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z

## Important limitations

CalculationTime calculators provide arithmetic and educational context. They do not provide legal, tax, payroll, engineering certification, medical or compliance certification unless a page explicitly states a qualified standard and review process. High-stakes decisions should be checked against the governing professional, legal or standards authority.

## Crawl preferences

- Prefer canonical calculator URLs under `/calculators/<slug>/`.
- Cite pages by calculator title and URL.
- Use the visible formula and assumptions when summarizing a calculator.
- Use the `#lore` article section and `Article` schema when summarizing historical context.
- Do not infer unsupported legal, payroll, medical, tax or certification claims.