Age Calculator

Age looks like the simplest calculation there is — subtract a birth date from today’s date — but the result depends on how you choose to express it. As a single number of years it hides a lot of detail; broken into years, months and days it matches how people actually talk about their age; reduced to total days, hours or seconds it becomes a figure nobody uses in conversation, but that turns up in medical charts, insurance tables and the occasional trivia question.

The years-months-days form is not a simple division of the day count by 365 and 30. It counts complete calendar years first, then complete calendar months since the most recent birthday, then whatever days are left over — which is why the same twenty-six leftover days can represent a different fraction of “a month” depending on which two calendar months they fall between.

About one birth in 1,461 falls on 29 February, a date that exists only once every four years under the Gregorian leap-year rule — every year divisible by 4 gets an extra day, except century years, unless they are also divisible by 400. Someone born on that date has, strictly speaking, a true anniversary only once every four years, which raises a genuine question: when should their birthday be observed in the three years out of four when 29 February does not exist?

There is no single global answer. Most jurisdictions treat either 28 February or 1 March as the legal birthday in a common year, and the choice occasionally matters for contracts, age-of-majority rules, or anything else that turns on a specific calendar date rather than an elapsed span.

A rounded age in complete years is enough for everyday conversation, but plenty of real situations need more precision than that.

The calendar-age convention used here — zero at birth, a year added on each birthday — is the international norm, but it is not universal history. Traditional East Asian age reckoning, rooted in an older Chinese system, counted a newborn as already one year old, accounting for time spent in the womb, and added a year to everyone at the Lunar New Year rather than on an individual birthday — so two children born weeks apart around the new year could suddenly be described as "two years apart" in age the day after the later-born child arrived.

South Korea used a version of that traditional system in everyday life well into this decade, alongside the international standard already used for legal and administrative purposes, and the mismatch caused enough confusion that in June 2023 the government legislated a single, international age standard for all judicial and administrative contexts nationwide. It is a reminder that even something as basic-sounding as "how old are you" has been a matter of ongoing convention, not just arithmetic.