🎂 Age Calculator

One Birth Date, Several Official Ages

An exact-age result follows the international convention: zero at birth, plus one on each anniversary. Other legal systems have counted differently, and some still do. South Korea long ran three reckonings side by side: "Korean age," under which a newborn is 1 and everyone gains a year on 1 January; "year age," the current year minus the birth year; and international age on official paperwork. Amendments passed on 8 December 2022 took effect on 28 June 2023 and made international age the default reading of laws and contracts that do not say otherwise. Birth-year counting was kept where cohorts matter. Military service obligations still follow it, children start elementary school in the March after the calendar year in which they turn six, and everyone born in the same year may buy alcohol and tobacco from 1 January of the year they turn 19.

Japan formally adopted international counting in 1902 with its Act on the Calculation of Ages, still in force, and one detail of it still sorts schoolchildren. The act applies the Civil Code's rules for computing periods, so a year of age is completed at the close of the day before the anniversary. A child born on 1 April therefore turns six at the end of 31 March. Japanese school year groups run from 2 April to 1 April, so children born on 1 April start first grade a full year ahead of children born a day later.

The Anniversary That Does Not Exist

English common law once leaned the same way: a person was treated as reaching an age at the first moment of the day preceding the birthday. For England and Wales, section 9(1) of the Family Law Reform Act 1969 replaced that rule, fixing the moment at "the commencement of the relevant anniversary of the date of his birth." The older rule survives elsewhere: United States Social Security regulations still fix attainment of an age at "the first moment of the day preceding the anniversary of his birth."

The statute does not say what the relevant anniversary is when the calendar has no matching date. 29 February is about one calendar day in 1,506, and someone born on it has no exact anniversary in most years. Practice splits between 28 February and 1 March; the calculator uses 1 March. Someone born on 29 February 2000 reads 17 years, 11 months and 30 days on 28 February 2018, then a flat 18 years the next day. The birthday countdown also points to 1 March in common years.

Where the Arithmetic Involves a Choice

The years figure is unambiguous; the months-and-days remainder is not. Whenever today's day of the month is earlier than the birth day of the month, the calculation has to borrow the length of some month, and no universal rule says which one. Take someone born on 31 January and checked on 30 March in a common year. Borrowing February's 28 days gives "1 month and 27 days," while borrowing January's 31 gives "1 month and 30 days." The elapsed time is 58 days either way. The calculator borrows the length of the month before the current one, so the result can differ by up to three days from tools that borrow from the birth month.

Hour-level figures rest on a starting assumption. Because you enter a date rather than a time, the calculator counts from midnight at the start of your birth date, in your device's current time zone, up to your device's clock. The hour count, and the heartbeat and breath totals built from elapsed minutes, therefore run from that midnight. The later in the day you were born, the more they overstate, by up to nearly a day. If you were born in another time zone, the offset can push them either way. The day total counts completed 24-hour periods since that midnight, normally the same as the calendar days you have lived. If a daylight-saving change falls between your birth date and today, the two can differ by one during the hour around local midnight. To match a recorded birth time, subtract the hours between midnight and that time.