Methodology & Calculation Standards

1. Calendar-Based Normalization

All calendar-based tools use JavaScript Date objects normalized to UTC for day-level calculations. Exact age and date-difference modes strictly respect real month lengths (28, 29, 30, or 31 days) and Gregorian leap years rather than approximating every month as 30 or 30.4375 days.

2. Boundary & Leap Day Handling

Calculations adhere to rigorous boundary testing, including leap days (February 29), month-end adjustments (e.g., August 31 to September 30), reversed date ranges, milestone boundaries, and threshold transitions. For general calendar calculations, standard anniversary rules recognize February 28 as the anniversary in common years (clamping safely to the last day of February). Specific jurisdictions or legal statutes (such as under certain UK laws) may recognize March 1; check the governing rule for official decisions.

3. Estimation & Professional Disclaimers

Specialized calculators (such as biological age, gestational age, or financial service life) clearly document their underlying mathematical or clinical models. Results are informational and should not be treated as clinical diagnoses, legal determinations, or official actuarial certifications.

4. Client-Side Privacy & Performance

All calculator operations execute locally in your browser with zero latency. Date inputs are processed in-memory and never transmitted to remote servers.

5. How Tools Are Verified

Every calculator is checked against worked examples whose answers are produced independently of the tool itself — by hand calculation or from the governing reference — before the page is published, and the examples printed on each tool page remain part of that check: if a tool and its printed example ever disagreed, the page would be treated as defective until they matched again. A fixed set of boundary inputs is re-run whenever shared calculation code changes: births on 29 February, intervals starting on the 31st of a month, spans that cross a leap day, reversed date order, and identical start and end dates. One example from the set: a child born on 14 May 2019 and tested on 3 February 2026 is 6 years, 8 months and 20 days old; and a child born on 29 February 2020 turns exactly 6 on 28 February 2026 under the clamping rule in section 2, having been 5 years, 11 months and 30 days old the day before.

6. Rounding & Display Conventions

Ages and intervals are reported in completed units: a person is 6 years old from the sixth birthday until the seventh, and a partial month or day is never rounded up into the count. Where a method is genuinely decimal — decimal age, for instance — the tool page states the average it divides by (a mean year of 365.25 days, a mean month of 30.4375 days) instead of hiding it, because the calendar modes on this site never use those averages. Countdowns report whole days remaining at date level, and percentages produced by estimation models are shown rounded, with the inputs left visible so the estimate can be re-checked.

7. Sources Policy

Calendar arithmetic needs no external authority: the Gregorian rules are the standard. Domain conventions are different. Where a calculator implements a rule owned by someone else — an assessment publisher's age format, a pension-age timetable, an exam board's cutoff date, a clinical dating convention — the method is taken from that authority's own published material, and the accompanying guide names the source, the jurisdiction it applies to, and the dates it covers. Where authorities disagree, the calculator follows the general calendar rule and the page flags the exception — as section 2 does for 29 February anniversaries — rather than silently picking a side.

8. Corrections & Updates

Errors can be reported through the contact page. A report is first reproduced against the documented method. If the tool is wrong, the tool is fixed and every worked example that relied on the wrong value is corrected with it; if the tool is right, the reply explains the convention that produced the result. Time-sensitive content — exam cycles, pension timetables, holiday calendars — is reviewed when the underlying official notification changes rather than on a fixed schedule, and changes are made in place, so the live page is always the current version.

9. Limitations

Three limits apply across the library. First, calculations are date-based: times of day and time zones are excluded by design, so results are exact at day level and only approximate for intervals defined by timestamps. Second, estimation models — wellness ages, pet-age equivalents, lifetime totals — return the output of their stated assumptions, not measurements; change an assumption and the answer changes, and the tool pages say so. Third, business-day results depend on the holiday calendar that applies; where a tool does not ask which calendar applies, it counts weekdays only and leaves holidays out entirely.