How to Calculate Age Exactly
Counting full years is easy until a birthday falls on 29 February, a month has no 31st, or a law says an age is reached on a different day. This guide explains how exact age is counted and where the rules of different countries meet.
Years, Then Months, Then Days
An exact age is counted in completed units from the day of birth: first full years, then full months since the last birthday, then the days since the last monthly date. Someone born on 15 May 1990 is, on 13 October 2026:
- 36 full years (the 36th birthday was 15 May 2026);
- 4 full months since then (15 June, July, August and September);
- 28 days since 15 September.
The same span is 436 months and 28 days, 1,900 weeks, or 13,300 days. The age calculator shows all of these for any date, together with the day of the week of the birth and the next birthday.
Months That Are Too Short
Counting months runs into a problem when the day of birth does not exist in a month: someone born on 31 January has no 31 February. There are two ways to handle it. The month can be complete on the 1st of the following month, or on the last day of the short month. The calculator uses the same rule for months as you choose for 29 February birthdays, so the two agree.
Birthdays on 29 February
In common years, a person born on 29 February turns a year older on either 28 February or 1 March. The laws of Korea and Japan lead to 1 March:
- Korea: the Civil Act counts age from the day of birth (article 158). A period of years ends the day before the corresponding date, and when that date does not exist in the last month, on the last day of that month (article 160). The year therefore ends with 28 February, and the age goes up on 1 March.
- Japan: the Act on the Calculation of Age applies article 143 of the Civil Code, which ends a period on the day before the corresponding date, or on the last day of the month when there is none. Again the year ends with 28 February.
Other places and organisations follow their own rules, and many people simply celebrate on 28 February. Choose the rule you need in the calculator.
Japan: An Age Is Reached the Day Before the Birthday
Because the period ends at midnight at the end of the day before the birthday, Japanese law treats that day as the day an age is reached. The Japanese government confirmed this in a 2002 answer to the House of Representatives: a person's age is added at midnight at the end of the day before their birthday. This matters for school entry: children who turn six between 1 April and 31 March start school the following April, which means children born from 2 April to 1 April of the next year share a school year. Someone born on 1 April reaches the age on 31 March and starts school a year earlier than someone born on 2 April. On the birthday itself, the age is the same as everywhere else.
Korea: International Age Since 2023
Since 28 June 2023, Korean laws, contracts and official documents count age as international age, the same full years used elsewhere, unless a law says otherwise. Some laws still use year age (the current year minus the year of birth), and the traditional counting age adds one at birth and on every 1 January. The Korean age calculator shows all three; the Korean age guide explains when each one applies.
Age on a Specific Date
Schools, exams, sports leagues and benefits often fix a reference date, such as the start of the school year or 31 December, and decide eligibility by the age on that day. Enter that date as "Age on" instead of today's date. For legal deadlines, check the exact rule of the law or institution: some count the age reached during the year, and some follow the day-before rule.
Frequently Asked Questions
How many days old am I?
Count the days from your birth date to today. The calculator shows the total and the date you turn the next round thousand of days, such as 10,000 days old, about 27 years and 4 months after birth. It counts the day of birth as day 0; counting it as day 1 brings each date one day earlier.
Is age counted including the day of birth?
Yes. In Korea and Japan the law counts age from the day of birth itself. For most other periods the first day is not counted: Japan's Civil Code says so in article 140, and the Act on the Calculation of Age makes the exception for age.
When does someone born on 29 February legally turn 18?
Under the Korean and Japanese rules, on 1 March of the year they turn 18 when that year is not a leap year; in Japan the age counts as reached at the end of 28 February. Other countries may differ.
Sources
Source: 국가법령정보센터: 민법 제158조 · Last checked:
Source: 국가법령정보센터: 민법 제160조 · Last checked:
Source: e-Gov法令検索: 年齢計算ニ関スル法律 · Last checked:
Source: e-Gov法令検索: 民法 第143条 · Last checked:
Source: 衆議院: 年齢の計算に関する質問に対する答弁書（2002年） · Last checked: