Age calculator — exact age in years, months and days

Because 365.25 is an average, and an age is a count. There is no such thing as a year of 365.25 days or a month of 30.44 days — real years are 365 or 366 days and real months are 28, 29, 30 or 31, and which ones you crossed depends entirely on where in the calendar you started. Dividing gives an answer that is close but almost never exact, and the error is not evenly spread: it is largest for people born in the first three months of the year and smallest in the last. This calculator walks the real months instead. It counts how many whole months fit between the two dates, splits those into years and months, and reports whatever days are left over. That is the number that matches what a person would get by counting on a calendar, which is the only number worth printing.

Because the month that started on 31 January finished on the last day of February, not on 31 February, which does not exist. Working it through: the raw month difference between January and March is two, so the calculator first tries 31 January plus two months. That lands on 31 March, which is past the target, so the second month never completed and it is dropped. It then tries one month: 31 January plus one month clamps to the end of February — 29 February in 2000, 28 February in 1999 — and that is on or before the target. So one whole month has passed, and the days left over are counted from there: exactly one day. The answer is 1 month and 1 day in both a leap year and a common year, even though February had a different length. Testing the anchor against the target like this is the only way to get both the leap and the common case right; implementations that skip the test report 31 January to 28 February as one month, which would mean a month passed with a day still missing from it.

On 28 February, in this tool. That is a choice, not a fact — different jurisdictions have settled on different answers, some taking 28 February and some 1 March, and there is no convention that is correct everywhere. What matters is that a calculator picks one, applies it everywhere, and says which. The reason for 28 February here is internal consistency. The whole page is built on the end-of-month clamp described above, and under that rule 29 February 2004 plus 252 months is 28 February 2025 — so the years-months-days figure already reports exactly 21 years on 28 February 2025, with nothing left over. If the next-birthday panel said 1 March, the same screen would show "21 years, 0 months, 0 days" and "next birthday tomorrow, turning 21" at the same time. One rule governs both, so the page cannot disagree with itself. If you need the other convention for a legal or administrative purpose, put 1 March into the "as of" field and read the figures there.

Because eligibility rules are almost always written as thresholds — a minimum age, a maximum age, a band — and a threshold needs a single whole number to compare against. Someone who is 17 years and 11 months is 17 for that purpose, not "nearly 18", and the months are not a tie-breaker. Completed years is the first number this calculator shows, and it is the same number a form means when it asks for age. It also explains why the age changes on the birthday and not before it: the year completes on the anniversary, so the day before a birthday is still the previous age, no matter how close it is. When you need to know the exact date a threshold is crossed rather than whether it has been, use the turning-age table further down the page.

The calculator says so in a sentence and stops, rather than printing a negative age. This is a real case rather than a defensive one: it happens whenever a year is mistyped, whenever a two-digit year is entered and lands in the wrong century, and whenever the two dates are filled in the wrong order. Calculators that carry on produce output like "-3 years, -2 months" or, worse, a plausible-looking positive number obtained by taking an absolute value somewhere — which quietly answers a different question from the one asked. When the dates are the wrong way round here, the message also tells you how many days it is until the date of birth, which is enough to see immediately whether it was a typo or a due date.

They are exact conversions of the day count — calendar days multiplied by 24 and by 1,440 — and that is what people mean when they ask how many hours old they are. Two things they are not. First, no time of birth is asked for, so the figures are measured between dates rather than between moments; if you were born in the evening, the true elapsed hours are lower than shown by however many hours passed before you were born. Second, they are not wall-clock hours across a daylight saving change. In places that shift their clocks, two days of the year are 23 and 25 hours long, so a span crossing one of them contains slightly more or fewer actual hours than days × 24. Nothing in this tool stores a clock, which is exactly why the dates themselves are always right — but it does mean the hours figure is a calendar conversion, not a stopwatch reading.

Yes, including the century rule that most people forget. A year is a leap year if it divides by 4, except century years, which must divide by 400 — so 2000 was a leap year and 1900 was not, and 2100 will not be. All the arithmetic runs on the real Gregorian calendar rather than on an average year length, so every February the span crosses is counted at its true length. The range is year 1 to year 9999, using the proleptic Gregorian calendar, which is the modern calendar projected backwards. One caveat for anyone working with historical records: countries adopted the Gregorian calendar at different times between 1582 and 1923, so for a birth date before their local switch the arithmetic here is correct but may not match a date written in a contemporary document.

No. Nothing you type is uploaded or saved. The whole calculator is JavaScript running in this tab, with no server involved at any point, so it also works with the connection off. It deliberately writes nothing to your browser storage either — some tools on this site save drafts because losing a half-finished invoice hurts, but a date of birth is personal information and the answer takes one sitting, so there is nothing worth keeping and a reason not to keep it. Close the tab and it is gone. Like every other page here, the page itself carries advertising, which loads from Google and is explained on the privacy page.