Method, sources and limits
Every figure on this site is produced by a tested module from a named source, and every one of them stops being reliable somewhere. This page says where. Nothing on it is typed by hand — the ranges, counts and descriptions below are read out of the same code that produces the tables, so this page cannot drift out of date while the site changes underneath it.
What this site is
Age Reckoner answers how old someone is in the systems that disagree about it: the Gregorian calendar, the Hijri calendar, the East Asian traditional counts, the Hebrew calendar, the Solar Hijri calendar, and the orbital periods of the planets. It is two things sharing one set of calculations — 8 explainer pages written for a person, and 6 reference tables written for anything that reads.
The reference tables are the part worth citing. "You are 8,231 days old" is true for one person for one second. "Lunar New Year fell on 10 February 2024" is a fact that stays put, and this site publishes 199 years of them, 60 stem-branch combinations, 5 imperial eras to the day, and the month names of four calendars in their own scripts.
Where each answer comes from
| System | Method | Published for |
|---|---|---|
| Hijri (Umm al-Qura) | The Umm al-Qura calendar as implemented in ICU/CLDR and reached through Intl.DateTimeFormat. | 1882–2174 |
| Hebrew | The Hebrew calendar as implemented in ICU/CLDR and reached through Intl.DateTimeFormat. | 1500–2500 |
| Solar Hijri (Jalali) | The Solar Hijri (Jalali) calendar as implemented in ICU/CLDR and reached through Intl.DateTimeFormat. | 1800–2200 |
| Chinese lunisolar | The Gregorian–Lunar Calendar Conversion Tables published by the Hong Kong Observatory, ingested in full rather than recomputed. The lunar month begins on the day of the new moon at the meridian used for the modern Chinese calendar. | 1901–2099 |
| Planetary orbits | Sidereal orbital period — one full revolution around the Sun, measured against the fixed stars — taken from the NASA planetary fact sheets. An age is elapsed time since birth divided by that period. | Any date — an orbit is division, and has no tabulated range |
Where each one stops
| System | Limits |
|---|---|
| Hijri (Umm al-Qura) | Umm al-Qura is tabulated by the Saudi authority for 1300–1600 AH (≈1882–2174 CE); outside that range ICU extrapolates and this site declines to answer. Even inside it, Umm al-Qura is a printed civil calendar: a locally observed moon sighting may differ from it by a day, and practice varies by country. |
| Hebrew | The Hebrew calendar has been arithmetic (not observational) since roughly the 4th century CE, so conversions are exact rather than approximate. Dates are civil days: a Hebrew date properly begins at sunset the evening before, which this site does not model. |
| Solar Hijri (Jalali) | ICU uses the astronomical Solar Hijri calendar, whose year begins at the vernal equinox as observed at the 52.5°E meridian. Far outside the modern era, published tables and arithmetic approximations disagree with it by a day here and there. |
| Chinese lunisolar | Published for 1901–2099, the range HKO tabulates. Outside it this site declines to answer rather than extrapolate. The Vietnamese and Korean lunisolar calendars use their own meridians and can differ from this one by a day. |
| Planetary orbits | These are averages of orbits that are ellipses, not circles, and that perturb each other slightly; the periods are quoted to the precision the fact sheets give and no further. Nothing here is a calendar: no planet but Earth has months, a new year, or a date. A birthday on Mars is a figure of speech for a point in an orbit. |
Two limits are worth repeating outside a table, because they affect more people than the range checks do. A Hijri day and a Hebrew day both begin at sunset, not at midnight, and this site works in civil days: if a birth was after dark, the date shown for those two calendars is the day before the one most observance would use. And Japanese era years are published only from , when Japan adopted the Gregorian calendar; before that the official calendar was lunisolar and a Gregorian date does not map onto an era year without saying which conversion produced it.
How the numbers are kept honest
One implementation, never two. The tables on the reference pages and the answers in the calculator come from the same functions — a table cannot say one thing while a calculation says another, because there is only one of each. Every one of those functions is a pure function of its inputs with no clock inside it, which is what makes them testable at all.
The build refuses to publish a site that fails its checks. It type-checks, runs the test suite over the calendar modules, builds the pages, and then reads the built HTML back and asserts that every page contains the content that gives it a reason to exist, that every canonical URL agrees with the one place the domain is configured, that no page ships executable JavaScript, and that the sitemap and the routing name the same 16 pages in both directions. If any of that fails, the new build never replaces the running one.
Where a conversion could not be checked against an authority, the range is left unclaimed rather than guessed at. There are several such gaps and they are stated on the pages where they apply: Vietnamese lunar dates computed from Chinese tables, the Vietnamese zodiac animals that differ from the Chinese ones, and every date outside the ranges above.
Sources
- Chinese lunisolar dates — Hong Kong Observatory, Gregorian–Lunar Calendar Conversion Table, ingested 2026-08-18 and committed to this site's source, so a build never depends on that server being reachable.
- Hijri, Hebrew and Solar Hijri conversions — ICU/CLDR, reached through the
Intl.DateTimeFormatbuilt into the browser and into Node. No conversion arithmetic of this site's own. - Orbital periods — NASA Goddard Space Flight Center, planetary fact sheets, 9 bodies, sidereal periods.
- Time zones — the IANA time zone database, as carried by the runtime. City data covers 167 places today and is being replaced with a larger gazetteer.
Privacy
A birthdate is a piece of personal information and this site treats it as one by never receiving it. There is no account, no database and no server that sees your dates: the calculation runs entirely in your browser, and the only place a birthdate is ever written down is the URL, if you choose to construct one to share.
There are no cookies, so there is no cookie banner. There is no analytics script, no advertising, no affiliate link and no third-party request of any kind — the reference pages ship no JavaScript at all, and the pages load no fonts, no images and nothing from any other host. That is not a policy that could change quietly; it is checked by the build.
Corrections
If a date here is wrong, it matters more than a wrong date usually does: reference tables get quoted, and a wrong figure that has been quoted is very hard to withdraw. Corrections are genuinely welcome, and the more specific the better — the date, what this site says, what it should say, and where that can be checked.
Write to corrections@agereckoner.com.
This applies to the descriptions as much as to the arithmetic. Several of the traditions described on this site are living ones belonging to people who will read the page, and where the site has got a custom's framing wrong rather than its dates, that is worth an email too.
About this page
Every range, count and description above is read from the site's own source at build time — the 16 pages, the 199 lunar years, the ranges in both tables and the sentences describing each limit. If a calendar module's range changes, this page changes with it in the same build.
Prose last reviewed 2026-08-20. The site is agereckoner.com.