The Short Answer: Subtract, Then Check the Birthday
Your age in whole years is the current year minus your birth year, minus one more if your birthday has not happened yet this year. That is also how South Korea's Ministry of Government Legislation explained "international age" to the public when the country made it the legal default on June 28, 2023: subtract the birth year from the current year, and subtract one more if your birthday has not passed.
For an exact age in years, months and days, you go one step further and subtract each unit in turn.
The Borrow Method for Years, Months and Days
Write the two dates as year, month and day, and subtract from right to left, the way you would subtract numbers in school:
- Days: subtract the birth day from today's day. If that goes negative, borrow one month and add the number of days in the previous month.
- Months: subtract the birth month from today's month (minus any month you borrowed). If that goes negative, borrow one year and add 12.
- Years: subtract the birth year (minus any year you borrowed).
Borrowing "the number of days in the previous month" is the step that matters, because months run 28 to 31 days. It is why two calculators can disagree by a day near month ends: some borrow the previous month's length, some the birth month's. Neither is wrong; just use one rule consistently.
Worked Example: Born May 15, 1990, on October 21, 2026
- Days: 21 − 15 = 6 (no borrow needed)
- Months: 10 − 5 = 5
- Years: 2026 − 1990 = 36
Exact age: 36 years, 5 months and 6 days. Counting every day, including the nine February 29ths in between, that is 13,308 days, or 1,901 full weeks. The next birthday, May 15, 2027, is 206 days away.
Total Days Lived
For a count of days, do not multiply years by 365. Subtract the dates directly, which accounts for leap years automatically. Under the Gregorian rule published by the U.S. Naval Observatory, a year divisible by 4 is a leap year, except century years, which must be divisible by 400; so 2000 was a leap year and 2100 will not be. Someone born in 1990 has lived through the leap days of 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024.
Leap-Day Birthdays
If you were born on February 29, your exact birthday only appears every four years. In other years, software has to pick February 28 or March 1 as the anniversary, and the choice changes the answer for one day each year. Laws on when a leap-day baby legally reaches an age differ between jurisdictions, so for anything official, such as a license or a benefit, check the rule where you live rather than relying on a calculator.
Not Every Country Counted Age the Same Way
Counting from zero at birth and adding a year on each birthday is the international standard, but it has not been universal. Until 2023, everyday life in South Korea commonly used "Korean age", where a baby is one at birth and everyone gains a year on January 1. The revised Civil Act and Framework Act on Administrative Regulation, in force from June 28, 2023, made international age the default in law and official documents, which Yonhap News reported made Koreans "one or two years younger" overnight. Some exceptions remain that still count by calendar year, including school entry, military service, and the legal age to buy alcohol and tobacco.
Frequently Asked Questions
How do I calculate my age in years?
Subtract your birth year from the current year, then subtract one more if your birthday has not happened yet this year.
How do I calculate my exact age in years, months and days?
Subtract days, then months, then years. If the days go negative, borrow a month and add the number of days in the previous month; if months go negative, borrow a year and add 12.
How many days old am I?
Subtract your birth date from today’s date directly rather than multiplying years by 365, so leap days are included. Someone born May 15, 1990 is 13,308 days old on October 21, 2026.
How is age counted for someone born on February 29?
In non-leap years, calculators use either February 28 or March 1 as the birthday. Legal rules vary by jurisdiction, so check local law for official purposes.
What is the difference between Korean age and international age?
Under Korean age, a baby is one at birth and everyone adds a year on January 1. Under international age, which South Korea made the legal default on June 28, 2023, you start at zero and add a year on each birthday.
Sources
- Ministry of Government Legislation, Republic of Korea, briefing on the international age law, June 26, 2023 (korea.kr policy news).
- Yonhap News Agency, "New age counting system goes into effect without much disruption" and "Legal age for buying liquor, tobacco to stay the same despite new age-counting system", June 2023 (en.yna.co.kr).
- U.S. Naval Observatory, "Introduction to Calendars" (aa.usno.navy.mil/faq/calendars), Gregorian leap-year rule.
This guide is for education. For legal age questions, check the rules in your jurisdiction.