How age is calculated, including the awkward cases
You become a year older on your birthday, which sounds like a complete rule until the birthday is 29 February or the question is your age in days.
Age looks like the simplest date calculation there is: subtract the year of birth from the current year and adjust if the birthday has not come yet. That is the rule almost every country uses, and it is what the age calculator applies. The edge cases are where it gets interesting, and they come up more than people expect, in law, in software and in anything with an age threshold.
The basic rule
A person’s age in completed years is the number of birthdays that have occurred. Someone born on 15 June 2000 is 25 on 14 June 2026 and 26 on 15 June 2026. The age changes at the start of the birthday, not at the time of birth: the hour and minute of birth are ignored for every ordinary purpose, which saves everyone the question of which time zone a birth certificate is in.
In software the calculation is: take the current year minus the birth year, then subtract one if the current month and day are earlier than the birth month and day. Done on dates, it is exact. Done by dividing the number of days since birth by 365.25, it is wrong on about one day in four near each birthday, which is a common bug.
Born on 29 February
About one person in 1,461 is born on a leap day and has a birthday only every four years. For the age rule, the question is which day counts in the other three. Most legal systems have decided. In England and Wales a person born on 29 February reaches an age on 1 March in a non-leap year; the same is true in Hong Kong. In New Zealand and Taiwan the age is reached on 28 February. In the United States there is no federal rule and states differ. Software usually picks one convention or the other without saying; the tool here follows the 1 March rule and says so. For the leap years themselves, see the leap year guide.
Age in months and days
Parents of infants, doctors and anyone calculating a precise interval need age as years, months and days, and here a convention is unavoidable because months are not a fixed length. The usual method counts whole years first, then whole months from the last birthday, then the remaining days. From 31 January to 1 March is one month and one day in a non-leap year, because the month lands on 28 February (there is no 31 February) and one day remains. Different tools can legitimately differ by a day here, and the month arithmetic guide explains why. The date difference tool shows both the exact day count and the years-months-days breakdown so the two can be compared.
Age in days, weeks and hours
Total days are unambiguous: count the days between the two dates. Ten thousand days is reached at about 27 years and 4 months, and twenty thousand at about 54 years and 9 months. Age in hours is only well defined if the time of birth is known, and it should be computed in UTC: a baby born at 00:30 on 1 January in Tokyo was born on 31 December in London, and only an instant-based calculation gets the hours right.
East Asian age reckoning
Until recently, several East Asian countries used a system in which a person is one year old at birth and everyone becomes a year older on New Year’s Day, so that a child born on 31 December was two the next day. South Korea used this “Korean age” in everyday life alongside the international system in law until June 2023, when legislation made the international age standard for all official purposes. China, Japan and Vietnam moved to international age during the twentieth century, though the traditional count survives in some customs. When reading an age from a historical document from these countries, the convention matters by one or two years.
Thresholds and the day they are reached
Age limits in law are usually reached at the start of the birthday. A person born on 1 July 2008 may vote in an election held on 1 July 2026 in a country where the voting age is 18. Some jurisdictions define it differently, counting the day before the birthday as the day the age is attained, a rule inherited from English common law that held a person reached an age on the day before their birthday because the day of birth counted as a full day. Most countries have legislated the ordinary rule, but the old one still appears in a few statutes and in some contracts, and it is worth checking when a threshold falls on an exact date.
Age at a date in the past or future
Age on a given date uses the same rule with that date in place of today. Age at death, age at the time of an event and age on a future date all come from the same calculation and the same edge cases. The age calculator accepts any reference date, and because it works on civil dates anchored to UTC midnight, a daylight saving change in your zone cannot shift the result by a day.
What the calculators here do
Age is computed as completed years, with months and days by the convention above, and total days as a plain count. Leap-day birthdays use 1 March in non-leap years. No time of birth is asked for, because none of the common questions needs it, and the calculations are tested against known cases including leap days and the day before and after a birthday. Where a jurisdiction or a form uses a different convention, the numbers here can be adjusted by at most a day, and the page says which way.
Sources
Frequently asked questions
When does someone born on 29 February have a birthday in a non-leap year?
It depends on the jurisdiction. England, Wales and Hong Kong use 1 March; New Zealand and Taiwan use 28 February; the United States has no single rule. The age calculator here uses 1 March and says so.
Why do two age calculators give different months and days?
Because months have different lengths and the convention for counting a partial month is a choice. Whole years and total days are unambiguous; the months-and-days split can differ by a day between tools.
What was Korean age and does it still apply?
A system in which a person is one at birth and ages a year each New Year’s Day. South Korea made international age the legal standard in June 2023.
Does the time of birth affect age in years?
No. Age in years changes at the start of the birthday regardless of the hour. Only age in hours needs the time of birth, and that should be calculated in UTC.