How to use it
Set the date of birth first.
Leave the second field on today for a current age, or change it to any other date. Forms and eligibility rules usually care about age on a specific cut-off, not today.
Everything recalculates the moment you change a date.
Why not just a decimal
Nobody says they are 31.7 years old.
They say 31 years, 8 months and 12 days. Getting there means borrowing the way a calendar actually does. If the day of the month has not arrived yet, the calculator takes days from the previous month at that month real length, which might be 28, 29, 30 or 31. If the birthday month has not arrived this year, it drops a year and adds twelve months back.
That is how you would count it out loud. A decimal buries all of it under an average month length that does not exist.
The 31st of January problem
Ask what date falls one month after 31 January and there is no single right answer.
February has no 31st. Some systems clamp to the last valid day and give you 28 February. Others roll the overflow forward and give you 3 March. Both are defensible, and different software picks differently, which is why two age calculators can disagree by a few days on the same birthday.
This one clamps rather than rolls. Someone born on 31 January is one month older on 28 February, and in a leap year on the 29th.
You only ever notice this at month ends, which is precisely when it matters for a contract or a deadline. If a date is legally significant, check how the organisation counting it does the arithmetic rather than assuming.
Leap year birthdays
Roughly one person in 1,461 is born on 29 February. Three years in four, their birthday does not exist.
Different places settle this differently. Some treat 28 February as the birthday in common years. Others use 1 March. New Zealand and Taiwan have historically gone with 28 February, while the UK generally treats 1 March as the day someone turns a given age.
This calculator sidesteps the argument entirely by counting real elapsed days instead of picking a substitute date. Born on 29 February 2000? You are exactly 24 on 29 February 2024, and every day count in between is right whichever convention your family prefers for the cake.
South Korea got younger overnight
Here is a genuine example of age being a legal construct rather than a fact of nature.
Until 2023, South Korea ran a system where you were 1 year old on the day you were born, and everyone gained a year together every 1 January.
So a baby born on 31 December turned 2 the next day. Most Koreans carried two ages, sometimes three, depending on whether the context was casual, legal or administrative.
Then on 28 June 2023 the country switched to international age reckoning by law. More than 51 million people became a year or two younger overnight, at least on paper.
Not everything moved. Military service still uses year-based age, and the rules covering drinking and smoking kept their own reckoning, so a handful of regulations continue to run on the older system.
This calculator uses international age, the same as most of the world: zero at birth, one on your first birthday.
Frequently asked questions
Why does it say my birth date is in the future?
The birth date is later than the second date. Swap them, or move the second date forward.
Can I calculate age between two past dates?
Yes. Both fields accept any date, so you can measure the gap between two historical dates just as easily.
Are leap years handled correctly?
Yes. The calculation uses real month lengths including 29 February, and the total-days figure counts actual elapsed days.
What happens with a 29 February birthday?
The tool counts real elapsed days rather than substituting another date. Legal conventions differ on whether the birthday falls on 28 February or 1 March in common years.
Does the total-days figure include today?
It counts whole days between the two dates. On a birthday the day component resets to zero and the total keeps climbing by one a day.
Which age system does this use?
International age, meaning zero at birth. That is now the legal standard in South Korea too, following the change in June 2023.
Related: days between two dates · countdown to a date · pregnancy due date.