Age Calculator

Age looks like the simplest calculation there is until a form asks for it in years and months, or a school insists on the age as at the first of June, or someone is born on the twenty-ninth of February. Then it turns out that the obvious method — subtract the dates, divide by 365.25 — quietly gives a different answer to the one every official system uses.

This calculator counts the way a calendar counts. Whole years first, then whole months from that anniversary, then the days left over. It also gives the totals in months, weeks and days, says which day of the week the birth date fell on, and works out how far away the next birthday is.

The second date defaults to today but can be set to anything, so the same tool answers 'how old will she be when school starts', 'how long was I at that job' and 'how many days until the visa expires'.

Total days divided by 365.25 produces an average year, and nobody lives an average year. Leap days are not distributed evenly through a lifetime, so the result drifts by a day either way depending on where the birth date sits relative to them. For 'how old in whole years' it usually agrees; for anything more precise it usually does not.

It also cannot answer the question a form is actually asking. 'Three years and two months' is a statement about the calendar, not about a quantity of days: three years and two months from the 15th of January is the 15th of March, whether or not February had 28 days that year.

So this calculator never converts to days and back. It counts whole months forward from the earlier date until one more month would overshoot, splits that count into years and months, and reports the remaining days as a day count. The parts always rebuild the end date exactly, which is the property a divide-and-round approach cannot promise.

A leap-day birthday has a real anniversary only once every four years, so three years in four something has to be decided. The convention used here, and by most legal and administrative systems, is that the anniversary falls on the 28th of February: a person born on the 29th of February 2000 turns twenty-five on the 28th of February 2025, not the 1st of March.

The tool says out loud when it has applied that rule rather than leaving you to work out why the number is what it is. The same clamping applies to any date that does not exist in the target month — the 31st of January plus one month is the 28th or 29th of February, not the 2nd or 3rd of March.

One consequence worth knowing: because a month can be clamped, the days remainder can jump. From the 31st of August, the 28th of February reads as five months and twenty-eight days, and the 29th reads as six months exactly. That is inherent in calendar arithmetic, not a rounding error.

Any two dates work, and the second does not have to be later than the first — enter them the other way round and the gap between them is shown, with a note saying so. That covers notice periods, probation windows, project lengths, gestational dates, warranty cover and visa stays.

For anything with a legal deadline, check whether the rule you are working to counts the first day, the last day, or both. The total days figure here is the plain difference between the two dates: the number of midnights crossed. A rule that says 'within 90 days including the day of arrival' wants that figure plus one, and no calculator can guess which convention your particular form has in mind.

Today is read from your own device, in your own time zone, after the page has loaded. It is deliberately not filled in on the server: a server in one time zone and a reader in another disagree about the date for several hours out of every twenty-four, and a page that renders one date and then corrects itself to another is worse than one that waits a moment.

Everything after that is pure calendar arithmetic with no time component at all, so the answer does not shift with daylight saving, and it does not matter what hour of the day you ask.