International age
Increases on the birthday. Take this year minus the birth year, then subtract one if this year's birthday has not passed. Since June 2023 this is the default basis for law and contracts.
Three different ages
One person has three different ages on the same day, differing by up to two years. Documents showing different numbers is normal, not an error.
Korean counting age
You are one at birth and gain a year when the calendar year changes: this year minus birth year plus one. Someone born on 31 December turns two the next day, which is why law dropped it. It survives in conversation.
Year age
Simply this year minus the birth year, ignoring the birthday. Conscription and youth-protection law use it, and they were excluded from the 2023 unification.
Enter a birth date in the age calculator to see all three at once.
What the 2023 change actually did
What changed
From 28 June 2023, the Framework Act on Administration and the Civil Act state that age is counted as international age. Where a law or contract simply says "age N", it is read as international age. This settled a long-standing source of disputes.
Why is it still confusing?
What was unified is the default reading, not every statute. Where an individual law specifies year age, that law wins. Conscription and youth-protection law are the notable cases, and because those are the age rules people meet most often, it can feel as though nothing changed.
Why do alcohol and tobacco limits ignore birthdays?
Youth-protection law uses year age: eligibility starts on 1 January of the year you turn 19, regardless of your birthday. Everyone born in the same year gains it on the same day, so a December-born person is treated as an adult eleven months earlier than a January-born one.
What about school years?
School entry is grouped by birth year, so classmates share a birth year regardless of how age is counted. The 2023 change does not affect it.
Where the calculation actually goes wrong
The increase happens on the birthday itself
International age increases on the birthday, not the day after. In law the birth date is included, so the new age applies from midnight. When a contract or eligibility rule hinges on a birthday, that single day matters.
Born on 29 February
People born in a leap year have no birthday in common years. Practice treats 28 February as the birthday; deferring to 1 March would delay the age increase by a year in effect.
Do not derive it from a national ID number
The leading digits alone cannot distinguish a 1900s from a 2000s birth year, and the digit that resolves it is sensitive personal data. If you need an age, collect a birth date — safer and legally simpler.
Conventions for counting days
Beyond age, plenty of situations ask "how many days", and whether the first day counts varies by context.
Does the first day count?
Hotel nights, working days, event spans and training hours usually count the start day too. A "how many days left" calculation counts only the difference. From 1 March to 3 March is two days by difference and three inclusive. Check which the contract means.
How is D-day counted?
The target date is D-DAY, the day before is D-1 and the day after is D+1. Counting elapsed days instead — "how many days since" — treats the first day as day one, so the same date is both D-DAY and day one.
Business days and holidays
A deadline "in business days" excludes Saturday and Sunday. Lunar holidays move each year and substitute-holiday rules change, so they resist automatic calculation. The business-day feature in the calculator excludes weekends only — cross-check the official calendar.
Adding months
Adding one month to 31 January gives 28 February, since 31 February does not exist. Civil-law period calculation clamps to the last day the same way. A calculator that rolls over to 3 March is not following this rule, which matters for contractual deadlines.
Building age conditions into a service
Age limits and adult verification raise real questions about how to store the value and when to compute it.
Why you should not store an age
An age stored as a number at sign-up is wrong on the next birthday. Store the birth date and compute the age at read time — it is the only correct approach. A schema with an age column is usually a trace of this mistake.
Which time zone applies?
A server on UTC is still on yesterday before 9am Korean time. For a feature that flips on a birthday, users appear a year younger during those nine hours. Decide age in the user's local time, or at minimum pin an explicit service time zone.
Which age does a limit refer to?
Writing "14 or older (international age)" in the terms removes the ambiguity. Conversely, content covered by youth-protection law uses year age, so eligibility follows the birth year rather than the birthday. Mixing the two gives the same user different rights on different screens.
How should the date be entered?
<input type="date"> lets the browser handle locale formatting and the calendar UI, and the value always arrives as YYYY-MM-DD. Three hand-rolled select boxes will happily accept 31 February and need separate validation.
Summary
- In law and contracts, an age means international age, and it increases on the birthday itself.
- Conscription and youth-protection law use year age, which is why alcohol and tobacco limits change at the start of the year.
- Korean counting age was dropped from law and survives only in conversation.
- People born on 29 February are treated as having a 28 February birthday in common years.
- Check whether a period counts the first day. Hotel nights, working days and events usually do.
- Adding months clamps to the last day: 31 January plus one month is 28 February.
Use the age calculator to see all three ages and the date arithmetic on one screen, including D-day and date addition.