Age Calculator

Completed birthdays, calendar span, elapsed days

Completed birthdays is the number the law and most forms mean: a person reaches an age at the start of the anniversary of their birth, so on 15 January 2026 somebody born on 15 January 1990 turns 36 and is 36 from that morning. This is the integer the page leads with.

The calendar span is the more precise version of the same thing: 36 years, 8 months and 1 day on 16 September 2026. It is measured by stepping months through the calendar and then counting the leftover days, so that adding the span back to the birth date lands exactly on the measurement date. A span computed by dividing 13,393 days by 365.25 would be close and would not add back up.

Elapsed days is the number that needs no convention at all: 13,393 days between two calendar dates, no leap-year rounding and no month arithmetic. Hours and minutes are that number multiplied out, which is accurate for a document and a rough approximation of a life - nobody ages at a constant rate of 24 hours a day.

A day is not always 24 hours in the real world, and why that does not matter here

The page works in calendar days and applies no time zone. That is deliberate: an age is a property of calendar dates, not of instants, so 15 January is a birthday in Auckland and in Los Angeles, and a daylight-saving transition cannot make anybody a day older. Elapsed hours and minutes are exact multiples of 24 hours per calendar day; a clock-based calculation across a DST boundary would differ by an hour, and it would be answering a question nobody asked.

The consequence is that this age changes at midnight UTC rather than at the reader’s own midnight. For a birthday countdown that only matters in the last hour of the previous day, and the page states the dates it used rather than presenting a bare number.

Leap days: 29 February in a year that has none

Somebody born on 29 February has a birthday that exists only once every four years, and different systems resolve it differently. This page moves it to 1 March in a common year, following the convention used in English law, because 1 March is the day after the anniversary of the birth: the alternative, 28 February, celebrates the birthday before the date it actually falls on.

Because that is a convention rather than arithmetic, the page says when it has applied it. The milestone birthdays table shows the exact date chosen for each year, so a 29 February birthday can see "1 March 2027" and object to it if their own convention is different.

Ages that are not the same number everywhere

Counting age as completed years is the international convention, but it is not universal. The traditional Korean system counted a person as one year old at birth and added a year at each New Year, which could make somebody two years older than their international age; South Korea moved to the international system for official documents in June 2023. Chinese and Japanese traditional reckonings have their own rules, and some cultures count age from conception rather than birth.

This page implements exactly one convention - completed birthdays on the Gregorian calendar, measured at a stated date - and it says so, rather than pretending that "age" is a single unambiguous quantity. If a form asks for an age in a jurisdiction with its own rule, the rule is the authority, not this page.