Age Calculator

Enter a date of birth and this calculator returns the exact age in years, months and days, with the working shown. Add a second date to get the age on that day instead of today.

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How age is actually counted

Age is counted the way people say it out loud: whole years first, then whole months, then the days left over. Thirty-six years, two months and five days. That sounds simple, and for most of the year it is, but it hides a genuine problem that this page shows its working on.

Leave the second field blank and it uses today. Fill it in and you get the age on any date you like, which is what you want for a form asking how old somebody will be on the first day of term, or how old they were on a particular date.

How long is a month?

This is the awkward question underneath every age calculator, and there is no clean answer. A month is 28, 29, 30 or 31 days depending on which one it is. So when you count from the 31st of January, what is one month later? There is no 31st of February.

The tempting approach is to subtract the day numbers and borrow when the answer goes negative, the way you would with ordinary column arithmetic. It works most of the time and then fails visibly. Born on 31 January, measured on 1 March: the shortfall is 30 days, February only has 29 to lend, and the result comes out as one month and minus one day. Nobody is minus one day old.

So this calculator counts differently. It works out how many whole months have passed, steps that many months forward from the birth date to land on the most recent anniversary, and then counts the actual days from there. Because it measures forward from a real date rather than subtracting two day-numbers, the leftover days cannot come out negative. Born on 31 January and measured on 1 March, the anniversary lands on 29 February, and the answer is one month and one day.

Where a birth day is later than the target month is long, the anniversary is clamped to the last day of that month, and the result says so. Born on the 31st, your anniversary in a 30-day month falls on the 30th, because there is nowhere else honest to put it.

Worked example

Born 15 June 1990, measured on 20 August 2026.

That is 434 calendar months apart, and the 20th has already passed the 15th, so all 434 count. 434 ÷ 12 is 36 years and 2 months.

Stepping 434 months forward from the birth date lands on 15 August 2026. From there to the 20th is 5 days.

So: 36 years, 2 months and 5 days, or 13,214 days in total.

Born on February 29

Roughly one person in 1,461 is, and their birthday only exists every four years. Three years in four, the calendar simply does not contain the day they were born.

This calculator observes the birthday on 28 February in a common year, which means someone born on 29 February 2000 turns 26 on 28 February 2026. That is a convention, not a fact, and it is worth knowing that it is not the only one: some legal systems treat 1 March as the anniversary instead, on the reasoning that a full year has not elapsed until the last day of February has passed. The difference matters for exactly one day a year, and it can matter quite a lot on that day if the question is when somebody legally becomes an adult.

The age in whole years is the same under both conventions everywhere except that single day, so unless you are reading a statute, either answer will do.

Age in months, weeks and days

Every result also gives the total in months, weeks, days and hours, which is more useful than it sounds. New parents count in weeks for the first few months and then in months until about two, because that is how developmental milestones and vaccination schedules are written. Anyone filling in a form for a young child usually needs the age in months rather than years. And the total days figure is the one to use if you are comparing two ages precisely, because it is the only one of the four with no convention baked into it at all.

Your birthday, and which day it falls on

The result names the weekday you were born on and the weekday of your next birthday, along with how many days away it is. The weekday of a past date is not guesswork: the calendar is fully determined, so the day you were born is a fact that can be recovered exactly, leap years and all.

Different countries count differently

The international reckoning used here, where you are zero at birth and gain a year on each anniversary, is not universal and has not always been the norm. South Korea long used a system where a baby is one at birth and everybody gains a year together on 1 January, which could make a child born in late December two years old within a fortnight. Korea moved to international age counting for official purposes in June 2023, one of the more unusual pieces of legislation of recent years, since it made most of the country a year or two younger overnight.

Traditional East Asian reckoning of this kind still appears in informal use in several places. If you have ever been given two different ages for the same person and assumed somebody made a mistake, this may be why.

What this calculator does not do

It does not account for time zones or the hour of birth. Ages are counted in whole days from one calendar date to another, which is how every form, school and legal system counts them. If you need the exact elapsed time between two moments including hours and minutes, that is a different question and our time duration calculator answers it.

Frequently asked questions

How do you calculate age in years, months and days?

Count the whole months between the two dates, split those into years and months, then count the actual days from the most recent anniversary. Counting the days forward from a real anniversary rather than subtracting two day-numbers is what keeps the answer from going negative at month ends.

Why do some age calculators show a negative number of days?

Because they subtract the day numbers and borrow once, the way you would in ordinary column arithmetic. Born on 31 January and measured on 1 March, the shortfall is 30 days and February only has 29 to lend, so the result comes out as one month and minus one day. This calculator counts forward from the anniversary instead, so that cannot happen.

What is one month after January 31?

There is no 31 February, so it has to be the end of February: the 28th, or the 29th in a leap year. This calculator clamps to the last day of the target month and says so in the result. The same applies to any birth date after the 30th when the target month is shorter.

When does someone born on February 29 have a birthday?

This calculator observes it on 28 February in a common year, so someone born on 29 February 2000 turns 26 on 28 February 2026. That is a convention rather than a fact: some legal systems use 1 March instead, reasoning that a full year has not passed until February is over. The difference matters for exactly one day a year.

How do I work out an age on a specific past or future date?

Fill in the second field. Leave it blank and the calculator uses today, but any date works, which is what you need for a form asking how old somebody will be at the start of term, or how old they were when something happened.

How many months old is my baby?

The results give the total in months, weeks, days and hours alongside the years-months-days figure. Weeks are the usual unit for the first few months and months up to about age two, because that is how developmental milestones and vaccination schedules are written.

Why do I get a different age in some countries?

Because not everybody counts from zero. South Korea long used a system where a baby is one at birth and everyone gains a year on 1 January, which could make a child born in late December two years old within a fortnight. Korea adopted international age counting for official purposes in June 2023. Traditional East Asian reckoning still appears informally in several places.

Does the time of day or time zone matter?

Not here. Ages are counted in whole days from one calendar date to another, which is how forms, schools and legal systems all count them. If you need exact elapsed time including hours and minutes, the time duration calculator handles that instead.

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