How to Calculate Age in Years, Months and Days

Working out someone's age sounds easy until the month ends and leap years get in the way. This guide explains how age is counted in years, months and days, how a 29 February birthday is handled, how to find the next birthday, and the small choices that make two calculators disagree by a day.

The simple part: whole years

Your age in whole years is the number of birthdays you have had. If you were born on 15 May 1990, you turned 36 on 15 May 2026. On 31 October 2026 you are still 36, and you will turn 37 on 15 May 2027. Subtracting the years of the two dates only works if the birthday has already passed this year, which is the mistake most quick calculations make.

Adding months and days

To get months and days, find the last date that is a whole number of months after the birth date and still not past today. Count the full months, then count the days left over. For a birth date of 15 May 1990 and a date of 31 October 2026, the answer is 36 years, 5 months and 16 days: 5 months after 15 May 2026 is 15 October 2026, and 16 days later is 31 October.

Taken in other units, the same span is about 437 months, 1,902 whole weeks and 13,318 days. The weekday matters too. 15 May 1990 was a Tuesday, which a calculator can tell you from the date alone.

What happens at the end of a month

Months are not the same length, so a birth date such as 31 January has no exact match in February. The usual fix, and the one our age calculator uses, is to move to the last day of the shorter month. Someone born on 31 January turns one month old on 28 February (or 29 in a leap year) and then 1 month and 1 day old on 1 March in a common year.

Other calculators handle this differently. Some borrow days from the previous month, which can give an answer a day or two different at month ends. Neither is wrong. They follow different conventions, and only the month ends are affected. If an exact figure matters, such as for a form, use the rule the form states.

Leap years and 29 February

The Royal Museums Greenwich explains why leap years exist: the solar year is 365.2422 days long, which no calendar year can match, so an extra day is added to keep the months matching the seasons. A year is a leap year if it is divisible by four, except that century years must also be divisible by 400. So 2000 was a leap year and 1900 was not, and 2100 will not be.

People born on 29 February are sometimes called leaplings. Wikipedia's leap year article says that in common years they usually celebrate on 28 February or 1 March, and that the law differs by place. It reports, for example, that the legal birthday of a leapling in the United Kingdom is treated as 1 March. Our calculator counts the birthday as 28 February in years that are not leap years and says so on the page. If your purpose is legal, check which rule applies in your case.

Days until the next birthday

The next birthday is the first occurrence of the month and day that is today or later. If your birthday is today, the answer is 0 days, and the age you reach is the one you just turned. For 15 May 1990, counted from 31 October 2026, the next birthday is on 15 May 2027, which is a Saturday, 196 days away, and the age reached is 37.

Common mistakes

• Subtracting the birth year from the current year without checking whether the birthday has happened yet.
• Counting 30 days in every month. Months are 28 to 31 days, and it matters at month ends.
• Assuming a 29 February birthday falls on the same date everywhere. Different places and forms use 28 February or 1 March.
• Mixing in the time of day. Age is counted in whole calendar dates, so the time you were born does not matter for these figures.

How to use the calculator

Pick the date of birth. The second date is today unless you change it, which is useful for working out an age on a past date, for example when a form asks for your age on a particular day. The result shows the age in years, months and days, the totals in months, weeks and days, the weekday you were born, and the next birthday.

Sources

• Which years are leap years? (Royal Museums Greenwich)
• Leap year (Wikipedia)