Enter a date of birth, and optionally a date to measure to, for an exact age in years, months and days. You also get the total in weeks, days and hours, the day of the week you were born, and how long until the next birthday.
Whole years first, then whole months, then the days left over. Where the day of the month would go negative, a month is borrowed from the previous one - so 31 January to 1 March is one month and one day, not a fraction of anything. This matches how people describe age and handles both varying month lengths and leap years without special cases.
Someone born on 29 February has an anniversary only every four years. Most legal systems treat 1 March as the effective date in other years, which is the convention used here. A minority treat 28 February instead, so the completed-year count is the reliable figure either way.
The system here is the international standard, where you are zero at birth. Traditional East Asian age reckoning counted you as one at birth and added a year at new year, which could make a December baby "two" days after being born. South Korea moved to the international system in law in 2023.
Whole years are counted first, then whole months, then the remaining days. This matches how people describe age in conversation and handles month lengths and leap years correctly.
In non-leap years the anniversary is treated as 1 March, which is the most common legal convention. The calculator counts the completed year either way.
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