Age Calculator

Korea used three different age systems at once until recently. Counting age (세는 나이) started everyone at one year old at birth and added a year every January 1 — so a baby born on December 31 turned two the next morning, at two days old. Year age (연 나이) is simply the current year minus the birth year, and international age (만 나이) is the one this calculator uses: you are zero at birth and gain a year on each birthday. On June 28, 2023, the so-called Age Unification Act (만 나이 통일법) — amendments to the Civil Act and the General Act on Public Administration — made international age the default interpretation in Korean laws, contracts, and official documents unless a statute explicitly says otherwise. Important exceptions survive: school entry, military conscription, and the legal age for buying alcohol and tobacco still use year age, so everyone born in the same calendar year crosses those lines together on January 1. Japan went through the same transition much earlier. A 1902 law made Western-style age (満年齢) official, and a 1950 law urged citizens to stop using the traditional counting age (数え年) — although kazoedoshi still governs ceremonies such as Shichi-Go-San and the yakudoshi "unlucky years". In China, the traditional xusui (虚岁) also starts at one and increments at Lunar New Year, while official documents use zhousui (周岁), the international count. If a friend in Beijing says they are 25 by xusui, their international age may be 23 or 24 depending on the time of year.

Turning a raw day count into "X years, Y months, Z days" is not standardized, because months have different lengths. The common algorithm — the one this tool uses — subtracts years, months, and days separately, and when the day-of-month goes negative it borrows the length of the previous month. That borrowed length is where implementations diverge. Someone born on January 31 and checked on March 1 has been alive for 1 month and 29 days under one convention (borrowing February's 28 days) but 1 month and 1 day under another. Neither answer is wrong; they are different conventions, which is why a government form, an HR system, and two websites can each show a slightly different month-day breakdown for the same person. February 29 birthdays add a legal wrinkle. Roughly one person in 1,461 is born on a leap day and only has a true birthday every four years. Jurisdictions disagree about when leap-day babies age in common years: New Zealand deems the birthday to fall on February 28, while the United Kingdom uses March 1. This calculator rolls the birthday over to March 1 in non-leap years. Japan has the most surprising rule of all: under its age-calculation law, a person legally becomes one year older at the end of the day before their birthday. That is why children born on April 1 join the school cohort above children born on April 2 — they legally turn six on March 31, just inside the school-year cutoff.

Birth: 1990-05-15   Today: 2026-07-02

Step 1  Years : 2026 - 1990 = 36 (May 15 birthday already passed)
Step 2  Months: 7 - 5 = 2, but day 2 < day 15 -> borrow one month
Step 3  Days  : 2 + 30 (length of June) - 15 = 17
Result: 36 years, 1 month, 17 days

Total days lived:
  36 x 365            = 13,140
  + 9 leap days (1992, 1996, 2000, ... 2024) = 13,149
  + 48 days (May 15 -> Jul 2)                = 13,197 days

Wrong shortcut: 13,197 / 365 = 36.16 "years" drifts because it
ignores leap days — always subtract calendar dates instead.

Most day-to-day uses of age tolerate rough numbers, but several legal thresholds turn on the exact date. Korea's national pension claiming age is climbing in steps: those born 1961–1964 can claim at 63, and the threshold reaches 65 for anyone born in 1969 or later. The US Social Security full retirement age is 66 plus a sliding number of months for people born 1955–1959, and a flat 67 from the 1960 cohort onward. A one-year error on a form can shift benefits by thousands of dollars. Ages of majority and consumption ages do not line up across the four locales this site serves. Korea's civil majority is 19 in international age, but alcohol and tobacco follow year age — anyone whose 19th calendar year has begun may buy them from January 1. Japan lowered legal adulthood from 20 to 18 in April 2022, yet kept drinking, smoking, and public gambling at 20. China's majority is 18, and the US famously licenses driving years before it allows drinking at 21. Korean life insurers use yet another system, insurance age (보험나이), which rounds to the nearest birthday: from six months after your last birthday you are treated as one year older, which changes premiums. If a quote looks wrong, check whether the company computed insurance age rather than international age before assuming a mistake.

The most common manual error is dividing total days by 365 or 365.25 and reading the result as an age. It drifts: across a 36-year span there are nine or ten leap days, and a plain 365 divisor claims someone is already 36 several days before their real birthday. Subtract calendar dates to get ages; use total-day counts only for totals. A subtler bug affects software, including many web calculators. In JavaScript, the string 1990-05-15 is parsed as midnight UTC, but "today" is taken in local time. In any timezone west of Greenwich, that mismatch silently shifts the birth moment several hours earlier than intended and can knock the day count off by one near midnight or near a birthday. This tool computes everything on your device with its local clock, so a birth registered in another timezone can legitimately differ by a day — the FAQ answer about one-day shifts is this exact effect. Also remember that the months shown here are calendar months, not 30-day blocks. A newborn is "1 month old" on the same day-number of the following month whether that gap was 28 or 31 days, and pediatric milestones and prescriptions usually intend calendar months as well. None of this is legal advice: when age determines a deadline, a benefit, or eligibility, confirm the counting rule the institution itself uses before relying on any calculator, including this one.