How Age Is Calculated

The short answer

Age in years is the number of birthdays a person has had. Subtract the birth year from the current year, and subtract one if this year's birthday has not happened yet. The age calculator does this, and also gives the months and days.

By hand

1. Write the two dates as year, month and day, with the birth date first.
2. Subtract the day. If the result is negative, borrow the length of the previous month and take one month off.
3. Subtract the month. If the result is negative, borrow 12 months and take one year off.
4. Subtract the year.

For 15 January 2000 and 20 March 2025: days 20 - 15 = 5, months 3 - 1 = 2, years 2025 - 2000 = 25. The age is 25 years, 2 months and 5 days.

Leap day birthdays

A person born on 29 February has a real birthday only in leap years. In other years, some countries and organisations treat 28 February as the birthday, and others 1 March. The rule matters when someone reaches a legal age on a leap day birthday. The age calculator lets you choose, so check the rule that applies to you.

International and East Asian age

Almost everywhere, age is counted the international way: 0 at birth, and one more on each birthday. Traditional East Asian age counting gave a newborn the age of 1 and added a year at each New Year, so a person could be one or two years older than their international age. South Korea moved to the international way for legal and administrative purposes in June 2023, though the older style is still used socially by some people.

Excel and Google Sheets

=DATEDIF(A1,TODAY(),"y")     ' complete years
=DATEDIF(A1,TODAY(),"ym")    ' months after the last birthday
=DATEDIF(A1,TODAY(),"d")     ' total days
=INT(YEARFRAC(A1,TODAY()))   ' complete years, alternative

The "md" unit of DATEDIF, for the days after the last month, is known to give wrong results in some cases, so avoid it.

SQL

-- MySQL
SELECT TIMESTAMPDIFF(YEAR, dob, CURDATE()) AS age FROM people;

-- PostgreSQL
SELECT EXTRACT(YEAR FROM AGE(dob)) AS age FROM people;

JavaScript, Python and PHP

// JavaScript
function age(dob, on = new Date()) {
  let y = on.getFullYear() - dob.getFullYear();
  const m = on.getMonth() - dob.getMonth();
  if (m < 0 || (m === 0 && on.getDate() < dob.getDate())) y--;
  return y;
}

# Python
from dateutil.relativedelta import relativedelta
age = relativedelta(date.today(), dob).years

// PHP
$age = (new DateTime($dob))->diff(new DateTime('today'))->y;

Time of birth and time zones

Age is counted in whole days. The time of day of the birth, and the time zone, are not used, so a person turns a year older at the start of the birthday. For the exact time between two moments, use a date-time library.

Other ways of expressing age

• Age in years, with decimal years and birth-year-only ranges.
• Age in months, for babies and toddlers.
• Age in days, with 1,000-day milestones.
• Age difference between two people.
• Date of birth from age.

Related tools

For any two dates, use date difference, and read the guide to days between dates.